opam pin --dev-repo has a strange UX when pinning a package from a multi-package repository

[kit-ty-kate]

Noticed in aantron/dream#357 (comment)

In the link above the user expected opam pin dream --dev-repo to pin all the packages from the dream repo correctly.
Currently the pin command only pins one thing at a time unless given a URL.

I feel like this behaviour is not the greatest. Like users have to know in advance that an upstream repository has multiple package before using opam pin --dev-repo.
I think we should change the behaviour (with --cli compatibility of course), to pin all the packages from that repository.
However I'm not sure how to allow users to do the previous behaviour (only pin one specific package), maybe something like a new --single-dev-repo option?

[kit-ty-kate]

Note from dev meeting: we agree that --dev-repo can be not what users want by default. However a new subcommand (e.g. opam pin dev-repo) as well as removing --dev-repo could be a better change.

This new subcommand could have several options such as --single to have the previous behaviour, --install-all to install all packages from that repository (defaults to pinning all but installing only the one requested), …

We'll give more thought about it later