Introduce Subsystem Component Registration

[garethky]

This change adds a new component registration mechanism that allows modules to register named components with one or more subsystems. The registration happens before printer object creation so no access to Printer is required, limiting possible side effects of registration. This creates a way to do truly dynamic registration without having registries in code, special directories, or files with lists of components.

Modules that want to register a component expose a register_components() function e.g.:

def register_components(subsystem: SubsystemComponentCollection):
    sensors = hx71x.HX71X_SENSOR_TYPES | ads1220.ADS1220_SENSOR_TYPE
    for name, sensor in sensors.items():
        subsystem.register_component("load_cell_sensors", name, sensor)

Modules can get registered components at load_config \ load_config_prefix time with a call to Printer.lookup_components. e.g.

def load_config(config: ConfigWrapper):
    printer: Printer = config.get_printer()
    sensors = printer.lookup_components("load_cell_sensors")

Most of the PR is re-working Printer.load_object so that it can use a cache of loaded modules. The cache is built prior to any printer objects being created. Having the cache allows for all modules to be scanned for a register_components function which is called before the config file is processed.

[rcloran]

This isn't the right PR to change this, since you're mostly refactoring existing code here, but I don't think this is the best way of going about finding modules -- it'll miss .pyc, .so, and doesn't work for stuff like a ZipImporter. pkgutil.iter_modules looks like it'd be much more robust.

[garethky]

I will look at pkgutil.iter_modules. This is the PR to make that change, lets get it right the first time.

[rcloran]

Suggested change

	if subsystem_name not in self._subsystems:
	return {}
	return self._subsystems[subsystem_name]
	return self._subsystems.get(subsystem_name, {})

But see also my comment about DefaultDict, in which case you can just

Suggested change

	if subsystem_name not in self._subsystems:
	return {}
	return self._subsystems[subsystem_name]
	return self._subsystems[subsystem_name]

[rcloran]

Personal preference maybe, but consider collections.DefaultDict for registries like this. It has its problems, though... But it makes these check-and-set patterns a lot more concise.

[garethky]

This is very basic use case, so it should work fine.

[rcloran]

I don't know this code well and haven't stepped through all the ins and outs, so this may be a dumb comment, but ... it really looks like these should now no longer be globals, but instead instance variables populated during load_config.

[garethky]

Agreed. I was trying to keep the spirit of the existing code but making them local lookups makes more sense.