Formulary: map formula names to local bottle paths

[Rylan12]

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This PR adds some new functionality to Formulary to allow for refs to be mapped to each other. This mapping is set up using Formulary::map and is then used to substitute the reference passed to Formulary::factory. For example:

Formulary.factory "foo"        # error
Formulary.map "foo", to: "bar" # setup mapping
Formulary.factory "foo"        # returns the bar formula

At the moment, this is not used anywhere internally, but would be used in Homebrew/json if merged. Originally, I had opted to monkey patch Formulary to look for cached bottles and was a bit unwilling to add it to Homebrew/brew because it felt too specific to that tap. However, I rethought the process a bit over the weekend and am now planning on monkey patching the changes from this PR (essentially). Since these changes feel more general and don't need to be specific to Homebrew/json, I feel okay including them here.

If the consensus is that we don't want it to be added here unless we're using it internally, that's okay.

[BrewTestBot]

Review period will end on 2021-06-15 at 16:07:27 UTC.

[MikeMcQuaid]

I'm fine with this functionality being included and merged but it feels a bit generic to me. If this is for cached bottles specifically: let's make the naming and example documentation reference those specifically. A generic map function seems both undesirable and potentially dangerous/insecure.

[Rylan12]

Is this better?

[MikeMcQuaid]

Could it be used now if an environment variable is set?

[Rylan12]

Used how? What would the environment variable do? Right now there's no place where it makes sense to set up the mapping outside of Homebrew/json. So I don't think adding an environment variable would do much here. As soon as this and https://github.com/Homebrew/homebrew-json/pull/7 are merged, anyone using Homebrew/json would use this

[MikeMcQuaid]

Yes, much better naming etc. 👏🏻

[BrewTestBot]

Review period ended.

[Rylan12]

Ah, I see. Those suggestions make sense to me.

Just thinking ahead a bit, is this the same env variable you're anticipating will be used to control whether homebrew/core is allowed to be untapped?

[MikeMcQuaid]

Ah, I see. Those suggestions make sense to me.

Just thinking ahead a bit, is this the same env variable you're anticipating will be used to control whether homebrew/core is allowed to be untapped?

It could be, yes, so feel rename to rename it accordingly 👍🏻

[Rylan12]

I think I'll go ahead and merge for now. I have a work-in-progress PR for untapping homebrew/core, so we can figure out the env var name there. Since its all undocumented and unsupported I'm fine changing it.