cmd/go: use Bitbucket v2 REST API when determining VCS
The existing implementation uses v1.0 of Bitbucket's REST API. The newer
version 2.0 of Bitbucket's REST API provides the same information but
with support for partial responses allowing the client to request only
the response fields that are relevant to their usage of the API
resulting in a much smaller payload size.
The partial response functionality in the Bitbucket API is documented here:
https://developer.atlassian.com/bitbucket/api/2/reference/meta/partial-response
The v2.0 of the Bitbucket repositories API is documented here:
https://developer.atlassian.com/bitbucket/api/2/reference/resource/repositories/%7Busername%7D/%7Brepo_slug%7D#get