x/exp/cmd/gorelease: allow major version comparisons

[carnott-snap]

Currently gorelease does not allow comparing between major versions. E.g. (on v2.0.0) gorelease -base v1.0.0 yields:

gorelease: can't compare major versions: base version v1.0.0 does not belong to module path/to/module/v2

While I can understand that this is not part of the normal release flow, it is incredibly useful for generating changelog data, or just figuring out what changed between two releases. Furthermore this is annoying to do with apidiff directly, because it requires NEW be discoverable in the main module:

git checkout v1.0.0
apidiff path/to/module path/to/module/v2

/cc @jayconrod

[jayconrod]

This is a subset of #39666, which should be fixed by CL 238839 as soon as that's reviewed.

Leaving this open to be fixed by another CL though since it would be handy to say -base=v1.0.0 instead of -base=example.com/mod@v1.0.0.

[gopherbot]

Change https://golang.org/cl/323856 mentions this issue: cmd/gorelease: allow -base to be a different major version