Pass original tap to formula when loaded from the API via TapLoader

[Rylan12]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

Fixes #16213

Follow-up to #16207

Supercedes #16215

This PR fixes the TapLoader refactoring to properly pass the old tap to the newly created formula, even though it is referencing the new formula file. This is consistent with the old behavior. This also fixes the newly-created issue where migrating to a different non-core tap but with the same name as a core tap incorrectly chooses the core formula.

Finally, this PR adds some more tests.

CC @MikeMcQuaid, @Bo98, and @amancevice

[Bo98]

This PR fixes the TapLoader refactoring to properly pass the old tap to the newly created formula, even though it is referencing the new formula file. This is consistent with the old behavior.

Makes sense to fix as it was before, but why does it behave this way? Seems really odd to have a formula in homebrew/core have a .tap point to something not-core.

[Bo98]

formula_name_path not returning the updated tap is very likely an error I made in 44f058e

[Rylan12]

Yeah, agreed it seems odd to intentionally use an old tap. I've pushed a fix. @amancevice this means your original fix was good. I've included it here for simplicity sake but have added you as a co-author. Thank you for submitting the original fix!

[amancevice]

very sporting of you @Rylan12 — cheers 🍻

[MikeMcQuaid]

Thanks @Rylan12!