Add a new CreateSoongInjectionDirFiles function

- This is a wrapper function for all files in soong_injection directory.
This should prevent an error in the workspace generated for
api_bp2build.
- Rename the existing CreateSoongInjectionFiles fn and make it package
  private to prevent confusion

(The subsequent CL in this stack should contain a smoke test for
api_bp2build)

Test: b build //:empty --config=api_bp2build --config=android
Test: TH
Change-Id: Iddb0aa1aff2f709826edd587aa99fccddf80f08f
diff --git a/bp2build/conversion_test.go b/bp2build/conversion_test.go
index dfc7f0b..43daddc 100644
--- a/bp2build/conversion_test.go
+++ b/bp2build/conversion_test.go
@@ -84,7 +84,7 @@
 
 func TestCreateBazelFiles_Bp2Build_CreatesDefaultFiles(t *testing.T) {
 	testConfig := android.TestConfig("", make(map[string]string), "", make(map[string][]byte))
-	files := CreateSoongInjectionFiles(testConfig, CreateCodegenMetrics())
+	files := soongInjectionFiles(testConfig, CreateCodegenMetrics())
 
 	expectedFilePaths := []bazelFilepath{
 		{