"Vendoring" scripts

[dwhswenson]

Currently, user must manually copy setup.py/version.py in order to vendor them. It would be nice to have scripts vendor-setup vendor-version that just output the relevant files. That would also allow us to automatically include a comment in the vendored files with the specific version of Autorelease that it comes from (making updates much easier on users).

I think these should just output to stdout, so the user can direct into whatever file they want. We definitely don't want to force them to overwrite existing files; imagine a case where a user vendored our setup.py, then needed to add customization, and later wanted to update changes that we've made. In this case, they can use the script to put the new code in new_setup.py, and then use a merging tool to select the relevant changes (barring massive rewrites, at least).

[dwhswenson]

For version.py this is trivial: inspect.getsource(autorelease.version). For setup.py, maybe the Autorelease setup.py will need to rewrite itself into the package as _setup.py? Or move the setup.py to autorelease/_setup.py and then create a new setup.py that just executes that file?

[dwhswenson]

Note: as of #78, it is already possible to vendor the GitHub Actions scripts with autorelease vendor actions, called from the root directory of the repo.