Simplify extension system

Finish the work started here: https://github.com/openmrs/openmrs-esm-core/pull/252 .

This simplifies the extension system. Previously, the extension system accommodated multiple slots having the same name. This was not part of the implementation plan and has no use case that isn't more simply implemented by using attach. This change removes the possibility of multiple slots having the same name, establishing a 1:1 correspondence between slots and names.

This also splits the extension store into an "internal" store and an "output" store, much the way the config stores are handled. This will enable module authors to get information about the extension system state while still permitting the extension system implementation to evolve.

This will make the implementation of the Link System much easier.