Tweaks for detection of git URLs (#530)

Tweaks for detection of git URLs (#530)

* Transform private repo URL into public one

For fetching from github, it is safe to do this transformation, whereas users
working with the repo may find more convenient the use of private git@ URLs.

* Detect github URLs missing a trailing ".git"

* Testsuite: more cases for git remotes

* Allow controlling the git@ -> https:// transformation

* Generalize checks beyond github

* Unit test for Alire.VCSs.Git.Transform_To_Public