x/exp/cmd/gorelease: fetch base version from directory with -basedir flag

[jayconrod]

gorelease currently lets the user specify a base version to compare against using the -base flag. At the moment, this only accepts an existing version of the same module (retrievable using go mod download). #37410 would allow version queries like @latest, @master, or @01234abcdef

It may sometimes be useful to compare against code that isn't available to the proxy or pushed to the origin VCS server yet. A -basedir flag would help with this. It would let the user specify the base version as a directory tree.

gorelease -basedir=path/to/base -release=v1.0.0

cc @liggitt