Allow tap migrations with renames.

[reitermarkus]

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This allows specifying full names in tap_migrations.json to migrate with a different name.

Also fix the old/new name in the warning messages.

[MikeMcQuaid]

Code makes sense, thanks! Can you supply a small example of what this PR enables in terms of migrations and whether it's for formula/casks/both?

[reitermarkus]

It enables this in tap_migrations.json:

{
  "vlc-beta": "homebrew/cask/vlc@beta"
}

We'll need it for migrating homebrew/cask-versions, but I added it for both casks and formulae.

[MikeMcQuaid]

We'll need it for migrating homebrew/cask-versions, but I added it for both casks and formulae.

Very nice 👍🏻 👏🏻

[Bo98]

Does this need extra migration code in update-report etc or is that covered?

[reitermarkus]

It should work with the existing code based on renames, since this basically only extends the list of renames.

[reitermarkus]

I haven't checked if we need to update the tap of installed casks somewhere, but I'm not sure if we even store this information like we do for formulae.

[Bo98]

iirc we do for JSON installs but not Ruby installs

[reitermarkus]

Yeah, so we don't have it for cask-versions. The question is do we need to install the .json during migration? Currently I think we only replace the token in the Ruby file.