System externals cleanup

[pearce8]

System externals should be sorted

Is there a utility to sort the yaml files alphabetically? Could we have a function to fix up a yaml file?

System externals can currently be stored in a yaml format, and also as a string in system.py.

These will be merged at system instantiation, but it is difficult for a human to know what this defines, and if there is more than one definition for a library, it is unclear which one would be used. Any suggestions?