diagnostic: use FromTapLoader to check 3rd-party formula

[cho-m]

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Might help with #16878 when name collision between existing core formula and 3rd party tap.

Seems to work locally for specific example.

❯ brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: Some installed kegs have no formulae!
This means they were either deleted or installed manually.
You should find replacements for the following formulae:
  packer

❯ brew doctor
Your system is ready to brew.

[apainintheneck]

Do we know if this is a regression and if so, how was it implemented before?

[MikeMcQuaid]

Do we know if this is a regression

Yes, it's a(another) regression from all the tap/formula/cask loader related refactoring work.

Personal preferences

[MikeMcQuaid]

Happy with this when you are @cho-m, please self-merge when you're ready and thanks for the quick turnaround on a fix here.