This looks really nice! I was looking for something like this recently, but ended up using
https://github.com/temoto/robotstxt. However, something that's closer to the Google version might well have made me choose that.
A couple of comments:
1) I always love packages that only depend on the Go standard library (and I know I'm not the only one who thinks this way). Your package pulls in ginkgo and gomego and x/tools, but they're only used for the tests. I think it'd be a selling point if it only depended on the stdlib's "testing" package. You could fairly easily rewrite these tests as table-driven tests and avoid those 3rd party packages.
2) I think a bit more cleanup would be good to make it a bit more Go idiomatic, for example merging the _cc and _h files into one (that's an artifact of C++).
Cheers,
Ben