Module extension usability improvements

[Fabian Meumertzheim]

Hi,

While working on bzlmod-ifying rules_go and Gazelle, I came across situations in which I found the API offered by module extensions to be somewhat more verbose and/or complicated than what WORKSPACE users are used to. 

More concretely, this led me to think about how the explicit use of `use_repo` and the need to "guess" the repository name used by the extension could be replaced by a more structured approach. I wrote up my ideas in this gist: https://gist.github.com/fmeum/c87c183ad6ffc55add0ca04232761ea1

Please let me know what you think about this. This is not meant to be a formal proposal yet, rather I just want to initiate a discussion on possible improvements to usability. It may not even be feasible to implement, @Xudong Yang @pcl...@google.com please let me know if that should be the case.

Best,

Fabian