Add a --download-only flag to "opam install"

[Armael]

This implements #4036 (cc @kit-ty-kate ).

This is still WIP because even when using the option, we get the usual message:

The following actions will be performed:
  ∗ install dune            2.1.3
  ∗ install conf-gmp        1      [required by z3]
...
===== ∗ 9 =====
Do you want to continue? [Y/n]

It would be better to recall explicitly at this point that in fact, this will only fetch the sources of the packages. I'm not sure how to do it though, in part because this message is currently printed using the graph before the build and fetch actions are made explicit, so we only have install/remove/reinstall/(change?) actions.

[rjbou]

Thanks for the pr!

For that, there is two way to proceed:

• update request type to handle fetch actions, but it is maybe too intrusive for just a download only option
• handle this case without calling all install mechanism, but by resolving the given request and downloading all its dependencies. A function can be added in OpamClient.

Another thing, on the behavior: If the package if already installed, install mechanism would end on a noop. In the meantime, an opam clean can have been launched, and its dependencies no more available locally. In this case, should it download them even if installed (and its deps) or assume that it is installed, no need to?

[rjbou]

Also related to #2895

[rjbou]

Finalised, just waiting got cli versioning PR to be merged