Support subtracting sources from globs

Support -file or -path/glob in file lists that contain globs to
subtract files from the resulting glob.  Also move source file
prefixing and handling into a common function.

Change-Id: Ib6d74ce22f53cae7348c4ba35b779976d90359a6
diff --git a/genrule/genrule.go b/genrule/genrule.go
index 589758d..1361631 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -67,9 +67,7 @@
 		Command: g.properties.Cmd,
 	})
 
-	srcFiles := g.properties.Srcs
-	srcFiles = pathtools.PrefixPaths(srcFiles, common.ModuleSrcDir(ctx))
-	srcFiles = common.ExpandGlobs(ctx, srcFiles)
+	srcFiles := common.ExpandSources(ctx, g.properties.Srcs)
 
 	g.outputFiles = make([]string, 0, len(srcFiles))