`opam-doc-collect` collects too many files

[samoht]

Some of these files are part of tests, or are hidden modules.

Ideally, we should install only the cm[td][,i] for the files the user really wants to install.

[lpw25]

The way to do this in the short-term is probably to check for a cmi with the same name and md5 has been installed somewhere in the "doc" switch before collecting it and its associated cmt/cmti and cmd/cmdi.