Opam file generation: support with-doc and post

[craigfe]

Opam currently supports five "package depends variables"¹:

1. dev
2. build
3. with-test
4. with-doc
5. post

Dune supports generating opam files with the first three, but not the last two. I've recently found post necessary when writing PPXs, so it'd be nice to see it supported by Dune.

This change could probably be folded in alongside solutions for #3653 and #3497.

Footnotes

1. Relevant opam code ↩

[dra27]

Note that global variables are also valid at this point (https://github.com/ocaml/opam/blob/master/src/state/opamPackageVar.ml#L20-L26) - in particular, opam-version, which I need for ocurrent/opam-dune-lint#26.

[emillon]

I'm not sure when this was fixed, but (depends (a :post)) is supported as of dune 3.3 - or any variable using the :name syntax. I just checked and this has been the case since the 1.10.0. But the docs suggest that only the first 3 are supported, so I'm tagging as such and will fix this.