[WIP] vcprompt from head-only to main tap #18234

Goal:
* move `vcprompt` back into main tap.

Done:
* supports `--HEAD`
* symlinks into CELLAR properly.
* home page link correct
* test works

TODO:

* Waiting on tagging, from upstream.  Request made at:

  https://bitbucket.org/gward/vcprompt/issue/5/tag-a-stable-10-or-other-release-so