x/exp/cmd/gorelease: positional arguments should specify packages to compare

[jayconrod]

Currently, gorelease compares APIs of all non-main, non-internal packages within a module. This is a sensible default.

In some cases, the API of a module's internal packages should be checked, since it's consumed by nested modules. For example, packages in cloud.google.com/go/internal are used in cloud.google.com/go/storage.

In other cases, non-internal packages should not be checked. For example, authors probably don't care about incompatible changes in packages in an experimental or unstable directory.

gorelease should accept a list of package paths or patterns as positional arguments. When specified, only these packages would be checked (main packages would still be excluded, since they can't be imported).