bp2build: Unexport GenerateBazelTargetsForDir.

Resolving a comment from aosp/1595678

Test: build/bazel/scripts/milestone-2/demo.sh
Change-Id: Ia7aba2b149fc09615fbafee67838632c71320f1b
diff --git a/bp2build/cc_object_conversion_test.go b/bp2build/cc_object_conversion_test.go
index 816edb8..f094102 100644
--- a/bp2build/cc_object_conversion_test.go
+++ b/bp2build/cc_object_conversion_test.go
@@ -167,7 +167,7 @@
 		}
 
 		codegenCtx := NewCodegenContext(config, *ctx.Context, Bp2Build)
-		bazelTargets := GenerateBazelTargetsForDir(codegenCtx, dir)
+		bazelTargets := generateBazelTargetsForDir(codegenCtx, dir)
 		if actualCount, expectedCount := len(bazelTargets), len(testCase.expectedBazelTargets); actualCount != expectedCount {
 			fmt.Println(bazelTargets)
 			t.Errorf("%s: Expected %d bazel target, got %d", testCase.description, expectedCount, actualCount)