Use of git repositories as pin remotes (#715)

Use of git repositories as pin remotes (#715)

* Skeleton function and calls from Commands.*

* Use a remote commit for a local pin

* Ensure remote pins are in place for build

* Show info for remote pins

* Fix url section separator @ -> #

* Generalize transitive links for remote links

* Display remote source in `alr pin`

* Refactor a few paths for reuse

* New test for pinned remote

* Document new feature

* Fix help for affected commands

* Tweaks during self-review

* Fix test for Windows

* Ensure cache is deletable on Windows