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