readall: read formulae that aren't tapped.

This will allow the reading and checking of formulae that mirror
Homebrew core formulae (e.g. `git` in a tap) and, with the previous exit
code changes, is a reasonable check for "are all the formulae in taps
updated to the latest core DSL".

Closes #28328.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>