Allow plugin classes to keep track of which plugin module provided them

Add support to plugin classes for recording which plugin module they were loaded from (and whether it was loaded from a filename or module name)

This way we can be more fine-grained about what plugins are actually being used when we output template config files.

For example, dnadna simulation init was always outputting:

plugins: ['dnadna.examples.one_event']

even if the one_event simulator is not being used. This can be extended to training templates as well, e.g. to only load the plugins needed for the network+transforms actually being used.

Follow-up work is still needed to do exactly that for the training config files.

This is a small part of #68 (closed).