cmd/uninstall: check for dependent casks

[nandahkrishna]

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This PR aims to add the ability to check for casks dependent on formulae when running brew uninstall, as requested in #10570.

Output:

➜ brew uninstall mono                 
Error: Refusing to uninstall /usr/local/Cellar/mono/6.12.0.107
because it is required by godot-mono, which is currently installed.
You can override this and force removal with:
  brew uninstall --ignore-dependencies mono

TODO: Comments for the new find_cask_dependents method and tests.

[BrewTestBot]

Review period will end on 2021-02-10 at 01:23:57 UTC.

[MikeMcQuaid]

Great work so far!

[BrewTestBot]

Review period ended.

[nandahkrishna]

I'm 👍🏻 on sharing code on them if it makes sense.

I've done this with the latest commit(s). I thought it would be better to have Keg.find_some_installed_dependents get dependents - irrespective of whether those dependents are kegs or casks (dependent on formulae). I hope my thinking isn't incorrect here.

I don't think there should be any references to casks in keg.rb though; casks never have/use kegs.

If I am wrong and/or this point (any references) takes precedence, I'll move the handling to cask/. I will admit I am new to the terminology so this is entirely possible.

[nandahkrishna]

With the latest commit, I've moved find_some_installed_dependents from keg.rb to a new installed_dependents.rb (hope I understood the suggestion right).

This method had quite a few tests and so I've moved them as-is from the previous keg_spec.rb, along with an additional test that is cask-related.

[MikeMcQuaid]

This is perfect, exactly what I had in mind. Great work @nandahkrishna 👏🏻 ❤️