Allow HOMEBREW_PREFIX replacement in external patches

[gaborbernat]

Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

This follows up the recommendation from #15841, to use:

patch do
  url "file://#{__dir__}/Formula.patch_name.diff"
end

for externally stored patches. My problem is that in this the following rule is not applied:

"In embedded patches, the string “HOMEBREW_PREFIX” is replaced with the value of the constant HOMEBREW_PREFIX before the patch is applied."

So I would like to enable this replacement (perhaps guarded by a boolean flag?).

What is the motivation for the feature?

This would allow us to move the embeded patches to external diff files.

How will the feature be relevant to at least 90% of Homebrew users?

Would make maintaining complex formulas with large diffs easier, and easier to configure formatter onto the formula files (as the formatter would not impact the diffs).

What alternatives to the feature have been considered?

Keeping the diff embeded means I cannot use formatter on the formula file.

[MikeMcQuaid]

Hi again @gaborbernat! Would review a PR for this. Might need to figure out how best to interpolate it or whatever.