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/cc/cc.go b/cc/cc.go
index a5af73e..f634697 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -525,8 +525,7 @@
 
 	buildFlags := ccFlagsToBuilderFlags(flags)
 
-	srcFiles = pathtools.PrefixPaths(srcFiles, common.ModuleSrcDir(ctx))
-	srcFiles = common.ExpandGlobs(ctx, srcFiles)
+	srcFiles = common.ExpandSources(ctx, srcFiles)
 	srcFiles, deps := genSources(ctx, srcFiles, buildFlags)
 
 	return TransformSourceToObj(ctx, subdir, srcFiles, buildFlags, deps)