Format tags so that the line can be split.
Add a tag clib:<lib> for potential dependence on a C lib.

"Potential" comes from the fact that we do not take care of the flags,
we collect all possible libraries.
Check that OPAM packages exist in all cases.

When a correspondence Findlib → OPAM is not found, we guess that the
OPAM package has the same name as the Findlib one.  We now in addition
check that this OPAM package indeed exists and issue messages
accordingly.