Support filegroups

filegroup is a module that contains a list of files, and can be used
to export files across package boundaries.  filegroups (and genrules)
can be referenced from srcs properties of other modules using the
syntax ":module".

Test: m -j
Change-Id: I3d6fc4819c0b4225b474e0ad42f0d947f55a5961
diff --git a/Android.bp b/Android.bp
index a941ca4..23cbad8 100644
--- a/Android.bp
+++ b/Android.bp
@@ -159,6 +159,7 @@
         "soong-android",
     ],
     srcs: [
+        "genrule/filegroup.go",
         "genrule/genrule.go",
     ],
     pluginFor: ["soong_build"],