The recently introduced CMP0200 improved configuration selection, but
did not correctly implement case-insensitivity. Add some additional case
smashing to ensure that matching is not case sensitive. Improve the test
which was passing due to falling back to the first listed configuration
rather than selecting a matching configuration.
Create a brand new implementation of `cmTarget::GetMappedConfig` which
prioritized a target's `IMPORTED_CONFIGURATIONS` as the 'source of
truth' for what configurations are available. In particular, this means
that configuration selection when `IMPORTED_CONFIGURATIONS` is set does
not depend on the library type in any manner. The fallback logic also
uses a more consistent 'usability' criteria that should result in more
consistent configuration selection, particularly for `INTERFACE`
targets.
The previous implementation is retained as a separate method for users
requesting the OLD behavior.
Fixes: #27022
Matthew Woehlke