Wrap lines of external-lib-deps hints

[gpetiot]

Hi, I customized the pretty-printer to make it easier to copy-paste the suggested command line given as a hint (that's something I always found frustrating).

The motivation is to replace:

Hint: try: opam install capnp-rpc-lwt capnp-rpc-unix cohttp-lwt-unix current
current_ansi current_docker current_git current_github current_rpc
current_web dockerfile git-unix opam-0install prometheus prometheus-app

with

Hint: try: opam install capnp-rpc-lwt capnp-rpc-unix cohttp-lwt-unix \
current current_ansi current_docker current_git current_github current_rpc \
current_web dockerfile git-unix opam-0install prometheus prometheus-app

[rgrinberg]

Hi. Please note that the Pp library is maintained in this repository. In dune, we only pull from upstream periodically and you should propose any API changes upstream first.

[ghost]

This is related to #3433 (which I just merged). I like the idea of still printing the command on multiple lines but using \ to quote newlines. @gpetiot if you rebase, you now have to update a single Utils.pp_command_hint function that is used consistently across the code-base.

As Rudi mentioned, we moved the development of Pp in a separate repository. It's not very obvious at the moment as Pp is still in src/stdune. This will become clearer once we "unvendor" our dependencies: #3575

BTW, I didn't know about pp_custom_break_hints but it seems like it's going to be useful to cleanup the makefile printer and a few other cases. So thanks for making me discover this function :)

[ghost]

@gpetiot you can copy the changes you did in the other repository here. Sorry for the multiple hoops here, we recently started extracting a few components of general interested outside of Dune, but the workflow is not properly finalised.

[rgrinberg]

I updated the pp library in dune. So it should be fine to rebase this PR.