feat(pkg): relocatable compiler support

[Alizter]

This PR adds a way for us to detect if a compiler in our "package universe" is relocatable. Knowing this means we can avoid the use of our toolchains functionality and instead build the compiler like a regular package.

The current chosen heuristic for knowing so is by inspecting if we have the reloctable-compiler meta-package available (for interoperability with David's relocatable opam repository) or by checking for the relocatable meta-package in opam-repository.

We add a test checking the checking behaviour and making sure that compiler-like packages are able to be cached in these cases.

To try it out for yourself, put the following in dune-workspace:

(repository
 (name dra27-relocatable)
 (url "git+https://github.com/dra27/opam-repository.git#relocatable"))

(lock_dir
 (repositories dra27-relocatable :standard))

Or use (ocaml (= 5.5.0)) relocatable in your package (depends).

Part of the work on:

• Relocatable compiler in dune #13229

[avsm]

@dra27 did this which is merged now (a relocatable package in opam itself). ocaml/opam-repository#29451

Does its existence let the heuristics here be replaced with a conflict check for that package instead?

[Alizter]

@avsm I've deleted my earlier rushed meesage.

The heuristic here is to detect the relocatable compiler during build time. That way we can decide whether or not to treat it as a "toolchain" or a regular package. This will have to stay around, and can now be expanded to use the relocatable meta package too.

We invoke this heuristic when interpreting a lock file which is currently configured by the user.

The question of forcing relocatable for all users is not handled in this PR however. That will follow once we have good user-support for OCaml versions older than 5.4, for which either David's repository or our overlays will handle.