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/cc/cc.go b/cc/cc.go
index a17108f..8bf1467 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -833,7 +833,7 @@
 		cc, _ := m.(*Module)
 		if cc == nil {
 			switch tag {
-			case android.DefaultsDepTag:
+			case android.DefaultsDepTag, android.SourceDepTag:
 			case genSourceDepTag:
 				if genRule, ok := m.(genrule.SourceFileGenerator); ok {
 					depPaths.GeneratedSources = append(depPaths.GeneratedSources,