bp2build: Unexport GenerateBazelTargetsForDir.

Resolving a comment from aosp/1595678

Test: build/bazel/scripts/milestone-2/demo.sh
Change-Id: Ia7aba2b149fc09615fbafee67838632c71320f1b
diff --git a/bp2build/testing.go b/bp2build/testing.go
index 2e59999..9774915 100644
--- a/bp2build/testing.go
+++ b/bp2build/testing.go
@@ -175,3 +175,9 @@
 		ctx.CreateBazelTargetModule(customBazelModuleFactory, myProtoLibraryProps, attrs)
 	}
 }
+
+// Helper method for tests to easily access the targets in a dir.
+func generateBazelTargetsForDir(codegenCtx CodegenContext, dir string) BazelTargets {
+	buildFileToTargets, _ := GenerateBazelTargets(codegenCtx)
+	return buildFileToTargets[dir]
+}