Support compilers pinned or from ocaml-variants

[panglesd]

The versions given to the packages needed to be more precise:

• ocaml-system needs a version given by ocamlc --version
• ocaml needs to remove both the ~ and the + suffixes.

I still need to add tests but this time the approach seems better to me.

[panglesd]

This should fix #97

[panglesd]

Here is what we could do:

• Only use cache for unpinned, ocaml-system/ocaml-base-compiler/ocaml-system. For other compilers, we do not use the cache.
• Also, we could override the corresponding ocaml compiler opam package (instead of overriding always ocaml-system). This would remove the need for overriding the ocaml package!

I think this solution would end up with a reasonable amount of binaries in the cache (even though I think the local cache size would never be a problem for users... cache "hit" would be more of a problem.). Also, I think it is OK for people using pinned or custom compiler package to wait a little bit to build the platform tools in those switches, they still get the "dependency-free" platform tools.

The opam commands to know the which are the compilers package installed and whether they are pinned can be:

$ opam list --has-flag=compiler --installed --short                  
ocaml-variants
$

and

$ opam list --has-flag=compiler --installed --short --pinned         
$

[panglesd]

I hope to have converged to a good design wrt custom compilers in #106. Closing this in favor of #106.