Load plugins without having to re-install Cylc Flow

[kinow]

Describe exactly what you would like to see in an upcoming release

Given that we now have plug-ins in the main loop after #3492 (thanks to @oliver-sanders), we now should be able to work on the plug-in loading.

At the moment it's loading the entrypoints from the cylc-flow module, with a specific name. So after creating a new plug-in, users would need to check out the specific version of Cylc Flow they have, update setup.cfg, and then re-install it.

Instead, we should be able to navigate the python path and load all the entrypoints that match a specific name. The name would need to contain a namespace too.

Additional context

What we have:

cylc-flow/cylc/flow/main_loop/__init__.py

Lines 312 to 321 in 506fb35

	def load(config, additional_plugins=None):
	additional_plugins = additional_plugins or []
	entry_points = pkg_resources.get_entry_map(
	'cylc-flow'
	).get('main_loop', {})
	plugins = {
	'state': {},
	'timings': {}
	}
	for plugin_name in config['plugins'] + additional_plugins:

What we could try:

https://packaging.python.org/guides/creating-and-discovering-plugins/
Pull requests welcome!