Add new API to convert absolute source paths to relative paths. (#930)

There are already APIs for getting a relative product path from an absolute source path, or getting a relative source path for an *existing* source file, but there were no APIs for getting a relative source path for a *new* source file.  Prefabs will need this ability to be able to correctly generate a relative source path inside the prefab file before the file has been saved.
The logic for relative source paths is a little bit tricky because the paths are relative to the watch folders, and the watch folders can be nested, with different priorities to explain which should take precedence.  The input paths can also include specifiers like "." and "..", which need to be reconciled before creating the final correct relative path.  The included unit tests test all of the tricky edge cases that I was able to identify.

Code/Framework/AzFramework/AzFramework/Asset/AssetProcessorMessages.cpp

@@ -308,6 +308,56 @@ namespace AzFramework
             }
         }
 
+        //---------------------------------------------------------------------
+        GenerateRelativeSourcePathRequest::GenerateRelativeSourcePathRequest(const AZ::OSString& sourcePath)
+        {
+            AZ_Assert(!sourcePath.empty(), "GenerateRelativeSourcePathRequest: asset path is empty");
+            m_sourcePath = sourcePath;
+        }
+
+        unsigned int GenerateRelativeSourcePathRequest::GetMessageType() const
+        {
+            return MessageType;
+        }
+
+        void GenerateRelativeSourcePathRequest::Reflect(AZ::ReflectContext* context)
+        {
+            auto serialize = azrtti_cast<AZ::SerializeContext*>(context);
+            if (serialize)
+            {
+                serialize->Class<GenerateRelativeSourcePathRequest, BaseAssetProcessorMessage>()
+                    ->Version(1)
+                    ->Field("SourcePath", &GenerateRelativeSourcePathRequest::m_sourcePath);
+            }
+        }
+
+        //---------------------------------------------------------------------
+        GenerateRelativeSourcePathResponse::GenerateRelativeSourcePathResponse(
+            bool resolved, const AZ::OSString& relativeSourcePath, const AZ::OSString& rootFolder)
+        {
+            m_relativeSourcePath = relativeSourcePath;
+            m_resolved = resolved;
+            m_rootFolder = rootFolder;
+        }
+
+        unsigned int GenerateRelativeSourcePathResponse::GetMessageType() const
+        {
+            return GenerateRelativeSourcePathRequest::MessageType;
+        }
+
+        void GenerateRelativeSourcePathResponse::Reflect(AZ::ReflectContext* context)
+        {
+            auto serialize = azrtti_cast<AZ::SerializeContext*>(context);
+            if (serialize)
+            {
+                serialize->Class<GenerateRelativeSourcePathResponse, BaseAssetProcessorMessage>()
+                    ->Version(1)
+                    ->Field("RelativeSourcePath", &GenerateRelativeSourcePathResponse::m_relativeSourcePath)
+                    ->Field("RootFolder", &GenerateRelativeSourcePathResponse::m_rootFolder)
+                    ->Field("Resolved", &GenerateRelativeSourcePathResponse::m_resolved);
+            }
+        }
+
         //---------------------------------------------------------------------
         GetFullSourcePathFromRelativeProductPathRequest::GetFullSourcePathFromRelativeProductPathRequest(const AZ::OSString& relativeProductPath)
         {

Code/Framework/AzFramework/AzFramework/Asset/AssetProcessorMessages.h

@@ -288,6 +288,45 @@ namespace AzFramework
             bool m_resolved;
         };
 
+        //////////////////////////////////////////////////////////////////////////
+        class GenerateRelativeSourcePathRequest : public BaseAssetProcessorMessage
+        {
+        public:
+            AZ_CLASS_ALLOCATOR(GenerateRelativeSourcePathRequest, AZ::OSAllocator, 0);
+            AZ_RTTI(GenerateRelativeSourcePathRequest, "{B3865033-F5A3-4749-8147-7B1AB04D5F6D}",
+                BaseAssetProcessorMessage);
+            static void Reflect(AZ::ReflectContext* context);
+
+            // For people that are debugging the network messages and just see MessageType as a value,
+            // the CRC value below is 739777771 (0x2C181CEB)
+            static constexpr unsigned int MessageType =
+                AZ_CRC_CE("AssetSystem::GenerateRelativeSourcePathRequest");
+
+            GenerateRelativeSourcePathRequest() = default;
+            GenerateRelativeSourcePathRequest(const AZ::OSString& sourcePath);
+            unsigned int GetMessageType() const override;
+
+            AZ::OSString m_sourcePath;
+        };
+
+        class GenerateRelativeSourcePathResponse : public BaseAssetProcessorMessage
+        {
+        public:
+            AZ_CLASS_ALLOCATOR(GenerateRelativeSourcePathResponse, AZ::OSAllocator, 0);
+            AZ_RTTI(GenerateRelativeSourcePathResponse, "{938D33DB-C8F6-4FA4-BC81-2F139A9BE1D7}",
+                BaseAssetProcessorMessage);
+            static void Reflect(AZ::ReflectContext* context);
+
+            GenerateRelativeSourcePathResponse() = default;
+            GenerateRelativeSourcePathResponse(
+                bool resolved, const AZ::OSString& relativeSourcePath, const AZ::OSString& rootFolder);
+            unsigned int GetMessageType() const override;
+
+            AZ::OSString m_relativeSourcePath;
+            AZ::OSString m_rootFolder; ///< This is the folder it was found in (the watched/scanned folder, such as gems /assets/ folder)
+            bool m_resolved;
+        };
+
         //////////////////////////////////////////////////////////////////////////
         class GetFullSourcePathFromRelativeProductPathRequest
             : public BaseAssetProcessorMessage

Code/Framework/AzFramework/AzFramework/Asset/AssetSystemComponent.cpp

@@ -202,6 +202,7 @@ namespace AzFramework
             // Requests
             GetUnresolvedDependencyCountsRequest::Reflect(context);
             GetRelativeProductPathFromFullSourceOrProductPathRequest::Reflect(context);
+            GenerateRelativeSourcePathRequest::Reflect(context);
             GetFullSourcePathFromRelativeProductPathRequest::Reflect(context);
             SourceAssetInfoRequest::Reflect(context);
             AssetInfoRequest::Reflect(context);
@@ -234,6 +235,7 @@ namespace AzFramework
             // Responses
             GetUnresolvedDependencyCountsResponse::Reflect(context);
             GetRelativeProductPathFromFullSourceOrProductPathResponse::Reflect(context);
+            GenerateRelativeSourcePathResponse::Reflect(context);
             GetFullSourcePathFromRelativeProductPathResponse::Reflect(context);
             SourceAssetInfoResponse::Reflect(context);
             AssetInfoResponse::Reflect(context);