While working on a new backend for OCaml to generate Erlang sources, I
found the need to read the .cmi file back into a `Types.signature`
value.

@Drup spotted that I was reading the file and pointed out this value was
already being read during the type checking process.

A quick check at the `Typedtree.module_coercion` showed us that it would
be difficul to extract the same information that is readily available in
the signature value.

This change will expose the signature value directly, so other backends
relying on this signature information do not need to do the extra work
of reading it again.

No change entry needed