find_package: Don't test <name>_DIR against ignored paths

Tweak `find_package` to not compare an already-specified `<name>_DIR`
against the set of ignored paths. This is a minor behavior change in
that, if a previously found package is in a location that is NEWLY
ignored (i.e. because the user modified the ignored paths since the
previous run of CMake), we won't throw out the old result. However, it
also means that a user specifying `<name>_DIR` takes precedence over the
set of ignored paths, which seems like the desired behavior.

Note that the current behavior was introduced in commit 11f97d1968
(find_package: Refactor CMAKE_[SYSTEM_]IGNORE_PATH, 2022-01-28,
v3.23.0-rc1~31^2) and appears to have been unintentional.