Extract internal/stub into separate repo

[abhinav]

Extracted into https://github.com/abhinav/stub-go. git-spice is GPL3, but as the sole author of this package, I am relicensing it to BSD-3 in that repository.

[abhinav]

This change is part of the following stack:

• 441 ◀

_{Change managed by git-spice.}