x/exp/cmd/gorelease: compare modules with different paths

[jayconrod]

gorelease should be able to compare packages in two modules with different module paths. This is useful for 1) comparing across major versions, especially for the first release on a new major version, and 2) comparing an original module and a fork.

This could be implemented by expanding the -base flag to accept a module path and a version.

gorelease -base=example.com/m@v1.9.9 -version=v2.0.0

[gopherbot]

Change https://golang.org/cl/238838 mentions this issue: cmd/gorelease: extract loading functions out of makeReleaseReport

[gopherbot]

Change https://golang.org/cl/238839 mentions this issue: cmd/gorelease: support comparison of modules with different paths

[gopherbot]

Change https://golang.org/cl/277116 mentions this issue: cmd/gorelease: support comparing replacement modules