Cloning/building packages given as ORG/REPO

[wilfwilson]

Similar to #35, but not the same:

If I specify eg GAP_PKGS_TO_CLONE as digraphs/digraphs, then the command rm -rf "$pkg"*, which translates to rm -rf "digraphs/digraphs"*, fails to delete digraphs-1.3.1.

If I also have GAP_PKGS_TO_BUILD as digraphs, then both versions of Digraphs get compiled. This wastes a lot of time.

[wilfwilson]

This is causing the Semigroups package CI to fail, see semigroups/Semigroups#815 (comment) and subsequent comments.

[fingolfin]

Note that one can even specify full URLs to clone, then it'll be similar.

AS it is, GAP_PKGS_TO_CLONE is a crutch, and perhaps we'd be better off eliminating it outright. Packages that need to clone (and possibly build) additional packages can easily achieve that by inserting a suitable extra step in their CI.yml.