cmFindBase: Track initial state with an `enum`

When logging a `WriteFindBaseEvent`, one of the triggers to log is the
change in the "found" status of a `find_` command. Track the difference
between "undefined", "found", and "tried before, but not found" so that
the transition can be detected reliably.

Co-Authored-by: Ryan Krattiger <ryan.krattiger@kitware.com>