Change bazel_module property to be of struct type instead of string

Test: Manual verification, and update to genrule_test.go
Change-Id: Ic507a35438be5261342a958f3a462a73739e2482

genrule/genrule_test.go

@@ -726,7 +726,7 @@ func TestGenruleWithBazel(t *testing.T) {
 		genrule {
 				name: "foo",
 				out: ["one.txt", "two.txt"],
-				bazel_module: "//foo/bar:bar",
+				bazel_module: { label: "//foo/bar:bar" },
 		}
 	`