x/tools/go/vcs: RepoRootForImportPath doesn't support private bitbucket repositories.

[freeformz]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   go1.6.2
2. What operating system and processor architecture are you using (go env)?
   darwin
3. What did you do?
   If possible, provide a recipe for reproducing the error.

vcs.RepoRootForImportPath("bitbucket.org/XXXx/privaterepo",true)

1. What did you expect to see?

The repo root.

1. What did you see instead?

Errors with https://api.bitbucket.org/1.0/repositories/XXXx/privaterepo: 403 Forbidden

This is because it tries to ping w/o any auth info. This info can be provided via .netrc and/or git. go get doesn't have a problem fetching a private repo though.