Prepend path separator to GOROOT prefix when checking for stdlib packages

[nkubala]

previously, this code was simply checking if the build.Default.GOROOT was a prefix of the target package path to see if the package was from the stdlib. this was failing for me, as the default GOROOT /usr/local/go was being used (since I don't have one set), but all of my packages lived under /usr/local/google/....

sadly, there's no better solution in the filepath package to check if a path is nested under another path.

[RJPercival]

Could you add a test case for this please?

[RJPercival]

It sounds like this is deceptively hard to do right: golang/go#18358

I suspect this change might cause a bug if the user does define $GOROOT and includes a trailing slash. Perhaps using filepath.Clean() on both paths would fix this? I'd like to see some test cases to get some confidence that this is an improvement.