lookup_package always looks to EXTERNAL_ROOT first

Adds following when doing find_package in lookup:

- Inserts EXTERNAL_ROOT in first position of CMAKE_PREFIX_PATH if not changing root
- do find_package
- undo change to CMAKE_PREFIX_PATH

This is the only way to ensure that a newly installed packages will be found.
Had an issue where enthought has its own old version of SWIG that shadowed ours.