The new gem registration and usage system

Merge from mainline (Rebase)

Gems/Prefab/PrefabBuilder/CMakeLists.txt

@@ -38,14 +38,16 @@ ly_add_target(
             Gem::PrefabBuilder.Static
 )
 
-ly_add_target_dependencies(
-    TARGETS
-        AssetBuilder
-        AssetProcessor
-        AssetProcessorBatch
-    DEPENDENT_TARGETS
-        Gem::PrefabBuilder
-)
+# the prefab builder only needs to be active in builders
+# use the PrefabBuilder module in Clients and Servers:
+ly_create_alias(NAME PrefabBuilder.Builders NAMESPACE Gem TARGETS Gem::PrefabBuilder)
+
+# we automatically add this gem, if it is present, to all our known set of builder applications:
+ly_enable_gems(GEMS PrefabBuilder VARIANTS Builders TARGETS AssetProcessor AssetProcessorBatch AssetBuilder)
+
+# if you have a custom builder application in your project, then use ly_enable_gems() to
+# add it to that application for your project, like this to make YOUR_TARGET_NAME load it automatically
+# ly_enable_gems(PROJECT (YOUR_PROJECT_NAME) GEMS PrefabBuilder VARIANTS Builders TARGETS (YOUR_TARGET_NAME) )
 
 if(PAL_TRAIT_BUILD_TESTS_SUPPORTED)
     ly_add_target(