Detect available packages from homebrew

[kit-ty-kate]

Even if it makes less sense than on Debian or other distributions, having opam detect which packages are available in homebrew might be still useful:

• users might have local brew remotes
• this helps the solver decide to avoid certain packages (e.g. camlp4 which isn't available in homebrew anymore)
• this leaves the door open to easily tell opam that some packages have been installed manually (using opam option depext-bypass / --assume-depexts / --no-depexts)

cc @patricoferris who got the issue with camlp4 on the upcoming macOS CI

[rjbou]

Thanks!

[patricoferris]

Thanks @kit-ty-kate! Unfortunately it seems that the command brew formulae doesn't also list the set of aliases (or there's no way to make it include those). It uses this items script with these parameters. This is unfortunate because some alias names are used in opam files for example:

• Sqlite3 in opam is an alias in Homebrew.

I was testing this in macOS CI and came across it whilst installing current_ocluster:

[ERROR] Package conflict!
  * Missing dependency:
    - current_ocluster → current >= 0.3 → sqlite3 → conf-sqlite3

But a brew search reveals:

$ brew search sqlite3
==> Formulae
sqlite3 ✔

The problem with brew search is that it takes quite a while (I think it hits the Homebrew API for searching). The maintainers of Homebrew discussed aliases a long time ago. I can't seem to find any PRs that added any kind of support for this.

[kit-ty-kate]

#4801 also shows us that this PR in its current state does not handle casks