Prepare for camlp4 separation

I propose to add virtual packages for camlp4 provided by the compilers. Compilers providing camlp4 have a base-camlp4 preinstalled package and a virtual camlp4 package is added to the repository. It depends on base-camlp4 and will be updated to have an 'or' dependency on the real package when ready.

This is related to issue https://github.com/ocaml/opam-repository/issues/1446

Notice, that it is not yet a good idea to add dependencies to camlp4 since, opam generated system.comp compiler descriptions does not contains base-camlp4. A temporary fix could be to add something like a fake camlp4 package installable with a compiler with version <= 4.01.