Simple plugin discovery

[forman]

Is your feature request related to a problem? Please describe.
Developers of a plugin for xcube must specify an entry point in their setup.py and must know the entry point's syntax.

Describe the solution you'd like
xcube should also discover plugin modules by module naming convention. All modules named xcube_{plugin_name} should be considered an xcube plugin named {plugin_name}. See https://packaging.python.org/guides/creating-and-discovering-plugins/ for details.

Describe alternatives you've considered
https://pypi.org/project/pluggy/