completions: fix zsh autocompletion issues

[nandahkrishna]

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This PR aims to improve the ZSH completions.

Earlier, conflicts weren't handled:

➜ brew livecheck
zsh: do you wish to see all 9397 possibilities (4699 lines)?

➜ brew livecheck --formula
zsh: do you wish to see all 9397 possibilities (4699 lines)?

➜ brew livecheck --cask
zsh: do you wish to see all 9397 possibilities (4699 lines)?

Now:

➜ brew livecheck
zsh: do you wish to see all 9397 possibilities (4699 lines)?

➜ brew livecheck --formula
zsh: do you wish to see all 5541 possibilities (2771 lines)?

➜ brew livecheck --cask
zsh: do you wish to see all 3856 possibilities (1928 lines)?

Also, this fixes the problem of only one formula/cask/named argument being suggested despite a command being able to take multiple named arguments. However, it isn't smart enough yet (to handle minimum/maximum number of arguments).

Note: So far, I've only handled "direct" conflicts related to named arguments. Some other (known) issues do exist, even with respect to handling conflicts. For example,

➜ brew --cache --build-from-source
zsh: do you wish to see all 9397 possibilities (4699 lines)?

(This shouldn't suggest casks.)

[BrewTestBot]

Review period will end on 2021-03-16 at 00:00:00 UTC.

[MikeMcQuaid]

Very cool, nice work!

[BrewTestBot]

Review period ended.

[Rylan12]

This is really cool. It doesn't work with commands like brew uninstall yet, though (I think because of the difference between formula and installed_formula). Would it be difficult to get that working in this PR?

[nandahkrishna]

I have an approach in mind, although it may not be ideal. I'll try to get it done in this PR.

[Rylan12]

Fantastic! Works great! Thanks for working on this, @nandahkrishna!