ShouldKeepExistingBuldFileForDir look up by dir

instead of scanning every entry in the map for prefix-match, search each ancestor of the dir

Bug: N/A
Test: bazel_test.go#TestShouldKeepExistingBuildFileForDir()
Change-Id: I7c9afa456cca5a70624ae1e9c9b96c43b0110c1e