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/genrule/genrule.go b/genrule/genrule.go
index bb78b1f..5a2ac84 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -96,6 +96,10 @@
 	return g.outputFiles
 }
 
+func (g *generator) Srcs() android.Paths {
+	return g.outputFiles
+}
+
 func (g *generator) GeneratedHeaderDirs() android.Paths {
 	return g.exportedIncludeDirs
 }