Refactor code for partitions c srcs

To support protos (and other srcs that generate sources), we need to
partition further. Separate out into a separate common function.

Bug: 200601772
Test: build/bazel/ci/bp2build.sh
Change-Id: I7bf4cd96fd9a9fca4ccb3c96f21a04303201f891
diff --git a/bazel/Android.bp b/bazel/Android.bp
index b68d65b..80af2bd 100644
--- a/bazel/Android.bp
+++ b/bazel/Android.bp
@@ -14,6 +14,7 @@
     testSrcs: [
         "aquery_test.go",
         "properties_test.go",
+        "testing.go",
     ],
     pluginFor: [
         "soong_build",