Export config as library Ocamlcompconfig

[lthls]

A draft proposal for the option of exporting the Config module as a library of its own, as discussed in #9842.

[dra27]

I had originally thought it could be included in ocamlcompconfig.cma as config.cmo with that being a strength to prevent linking ocamlcommon and this at the same time, but that's possibly cruel to Dune!

LGTM - just an update to the header in utils/config.mli advertising that it's also available as Ocamlcompconfig in ocamlcompconfig.cma and also that, although the usual caveat emptor for compiler-libs applies, the interface isn't expected to change in a breaking manner very often.

[lthls]

I decided to go with a copy instead of reusing the original Config name to prevent various kinds of conflicts:

• Config is a very common name, it likely occurs in a number of OCaml programs already.
• While linking against both ocamlcommon.cma and ocamlcompconfig.cma should be discouraged, if you build an executable that uses two libraries, one linking against ocamlcommon.cma and the other against ocamlcompconfig.cma you can't avoid it.

[lthls]

It was discussed in the developer meeting that draft PRs should be more explicit about why they are in draft mode only.
For this one, I put it in draft mode because it is here to support the discussion at #9842, and I'm waiting for the discussion there to converge before getting this one out of draft mode.

[dra27]

• While linking against both ocamlcommon.cma and ocamlcompconfig.cma should be discouraged, if you build an executable that uses two libraries, one linking against ocamlcommon.cma and the other against ocamlcompconfig.cma you can't avoid it.

My feeling for this was that this really should be for executables - if a library has some need for a module from compiler-libs, then it really ought to link compiler-libs? It seems potentially more risky to me to have an executable which could have 3 interpretations of the configuration (one from Config, one from Ocamlconfig and then of course the actual configuration!).

[dra27]

#10419's merged, so I think this can be closed.