Rewrite unused package names

[Leonidas-from-XIV]

This PR started small and then continued snowballing a little as it was growing.

What this does is when deduplication is disabled (opt-in) to determine which dependency should build which dune packages (by scanning OPAM files for dune build -p <name>), this info is saved into the lockfile.

From there, when pulling, the it will process dune files and rename public names that are not to be built to avoid clashing.

[emillon]

One trick I recently learned is that you can patch the dune file of a subdirectory without writing in that directory: you can mark that directory as data_only_dirs and use (subdir) to set the contents of the (virtual) dune file in there. I don't know if that applies to that case, but that can be useful if you don't want to change the contents of the unpacked tarball.

[Leonidas-from-XIV]

That's an interesting trick, but I can unfortunately see a number of problems with nested directories where I need to modify foo/dune but there is a foo/bar/dune that would has to stay the same (or be also moved up using subdir).

Rewriting it this way I can't easily ignore dune files in OCaml (and hoping they don't create issues). The cleanest way would probably be to collect all dune stanzas and dump them into a top level dune file but then I would also need to know which dune-files are actually being evaluated (e.g. no point to collect dune files from data_only_dirs which might actually break the build if collected). I feel this is getting dangerously close to a half-baked implementation of the rule-discovery logic of Dune itself.

But this does point at the necessity of eventually implementing such a feature in Dune itself.