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.bp

@@ -46,6 +46,7 @@ bootstrap_go_package {
         "android/defaults.go",
         "android/defs.go",
         "android/expand.go",
+        "android/filegroup.go",
         "android/hooks.go",
         "android/makevars.go",
         "android/module.go",
@@ -186,7 +187,6 @@ bootstrap_go_package {
         "soong-shared",
     ],
     srcs: [
-        "genrule/filegroup.go",
         "genrule/genrule.go",
     ],
     pluginFor: ["soong_build"],