Reapply "Refactor Formulary::loader_for."

[reitermarkus]

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Re-reverts #16684 and fixes Homebrew/homebrew-core#162946.

Also depends on the fix in #16688.

[MikeMcQuaid]

Looks ok to me! Hold off merging until you can be offline for 2-4 hours to fix any reported regressions please @reitermarkus.