Fix few issues with filegroups

Bug: http://b/64121881
Bug: http://b/78188880

- Allow filegroup's properties to be extended by a LoadHook
- Support a filegroup (':module') in a prebuilt's 'Srcs' property to
export files from a different path as the prebuilt's sources.

This change also includes a refactoring that moves genrule/filegroup.go
to android/filegroup.go so that FileGroupFactory is visible in
prebuilt_test.go.

Test: Test
https://android-review.googlesource.com/c/platform/development/+/469159
in clang-tools branch on Linux, Darwin.  Test regular build in
aosp/master.

Change-Id: I3ff6215ab2e62955f039fd1086c31f1bd50ebcf6

android/prebuilt_test.go

@@ -109,6 +109,19 @@ var prebuiltsTests = []struct {
 			}`,
 		prebuilt: false,
 	},
+	{
+		name: "prebuilt file from filegroup preferred",
+		modules: `
+			filegroup {
+				name: "fg",
+			}
+			prebuilt {
+				name: "bar",
+				prefer: true,
+				srcs: [":fg"],
+			}`,
+		prebuilt: true,
+	},
 }
 
 func TestPrebuilts(t *testing.T) {
@@ -125,6 +138,7 @@ func TestPrebuilts(t *testing.T) {
 			ctx := NewTestContext()
 			ctx.PreArchMutators(RegisterPrebuiltsPreArchMutators)
 			ctx.PostDepsMutators(RegisterPrebuiltsPostDepsMutators)
+			ctx.RegisterModuleType("filegroup", ModuleFactoryAdaptor(FileGroupFactory))
 			ctx.RegisterModuleType("prebuilt", ModuleFactoryAdaptor(newPrebuiltModule))
 			ctx.RegisterModuleType("source", ModuleFactoryAdaptor(newSourceModule))
 			ctx.Register()