cmd/go: fix handling of gopkg.in/macaroon-bakery.v2-unstable

When we added v2.0.0+incompatible we generalized the API
enough to make it easy to also accepting these gopkg-specific
v2-unstable suffixes. Do that.

Fixes #23989.

Change-Id: Ieabed11a5250c2999d73450c10b20f4c645ad445
Reviewed-on: https://go-review.googlesource.com/128901


Reviewed-by: Bryan C. Mills <bcmills@google.com>