opam tree: Allow packages with a specific version, directories or local opam files, as input

[kit-ty-kate]

Fixes #5493
Fixes #5287

Additional questions:

• do we want to support --recursive and --subpath ?
• do we want to support --with-version ?

my personal opinion for those is to say "we can merge this now, open an issue for those options and leave it for later"

---

TODO:

• Add some reftests

[kit-ty-kate]

Additional questions:

* do we want to support `--recursive` and `--subpath` ?

* do we want to support `--with-version` ?

thinking about it some more i think we should not support these option as their name are confusing and hits towards an action in opam-tree instead of in the underlying opam-pin action. I would advise for thinking about them again for 2.3.

[rjbou]

I think we should support --recursive && --subpath as they are supported by other commands that permit directory lookup for opam files (lint, upgrade, install, etc.).
For --with-version, it is a pin specific option, so we shouldn't support.

These command shortcuts, ie have access to opam install/lint . instead of opam pin . ; opam install/lint <pkgs>, ease a lot the usage, but complexify set of handled option. Plus usually it is only a subset of flags that are ported, so sometimes users expect to have such option but it is not available. It is not a simple subject.

[rjbou]

The global idea is great, it would simplify opam tree usage.
I've put some remarks on the code itself, some tiny optimisations, support of rec/subpath & tests.

[rjbou]

All good, thanks!