Incorrect artifact copy for some std libraries cause Merlin to fail to locate definitions

[voodoos]

A few modules of the Stdlib are copied from the otherlibs folder. Here are, for example, the rules for Unix:

https://github.com/ocaml-flambda/flambda-backend/blob/4e08295381a98ca7a267e639e202f7058dcf4558/ocaml/otherlibs/unix/dune#L44-L62

There are a few things that prevent Merlin to find the information it needs:

• The cmt files are not copied, these are critical for Merlin functionality (jump-to-definition and document)
• There is at least one typo in (unixLabels.mli as unix/unixLabel.mli): it should be plural everywhere

Similar changes should also be made to the other libraries that are copied to the stdlib folder.

cc @ddickstein