find_package: Improve support for CPS multiple inclusion

Keep track of CPS files we have imported in CMake's state, and use this
(instead of the prior, temporary work-around that used `<name>_CONFIG`)
as a guard for trying to import more than once from the same file. This
has two advantages; first, it is robust against finding the same package
name in different locations in alternating searches, and second, it
allows us to load additional appendices associated with a root package
that has already been loaded.

Fixes: #26731