remove the custom dnadna-plugin: URL scheme for loading schemas from plugins

...replace it with jsonschema-pyref

jsonschema-pyref is a package I wrote which implements a more powerful generic form of what dnadna-plugin: URLs did, which is to resolve (when using $ref) a schema from a dict defined in a Python module.

For this to work properly it's also necessary for our built-in schemas to have an $id (which is good practice anyways) so that relative refs (e.g. $ref: base.yml in some of the net schemas) can be resolved correctly.

This is an overall more "correct" use of JSON Schema $refs and less ad-hoc schema resolution.

For users this should not change anything, since this is all just about internal machinery, specifically how configurations for plugins are validated.