FindSWIG: reduce calls to SWIG exe, and clean up variables

If all SWIG cache variables (EXECUTABLE, DIR, VERSION) are set,
there should be no need to invoke the SWIG exe on a reconfigure. This
changeset reduces the number of calls to the SWIG executable, and
additonially prevents temporary `SWIG_xxx` values from propagating outside
the `find_package` call.

I also refactored the conditionals to be less nested (and thus more
predictable), including changing theh definition of SWIG_USE_FILE to
reflect that it's if and only if SWIG_FOUND is trure.