Detect feature branches written with a different format

[CyrusNajmabadi]

We currently detect feature/XXX branches, which we use to make npm tags like feature-xxx. This works well except for one significant problem.

Specifically, because of golang/go#30851 we are unable to reference these feature branches using go modules. This is problematic as there's no easy way to then have downstream repos (like @pulumi/aws) reference the latest built version from this branch. While we can reference teh package by commit, that's much more error prone and difficult to manage.

This PR simply allows us to actually write our branches like feature-xxx instead of just features/xxx. This is actually simpler as we simply use that branch name as is instead of having to convert it to this form (like we do with features/xxx=>feature-xxx).

With this, downstream branches can then do go get github.com/pulumi/pulumi@feature-2.0 and have it work properly.

[CyrusNajmabadi]

Going to move ahead with this to unblock my testing efforts.