Suggested by colin_m in https://github.com/ansible/ansible-rulebook/issues/571

Extract:

When developing filter plugins, passing --source-dir does not allow for local filter plugins, only local local source plugins. This means that the filter plugin has to be published in a collection to be tested.

Proposal implementation:
Ideally we should have just one flag to take both kind of plugins (source and filters) but that would be a breking change in the CLI interface, so I see following options:

1. Add a new flag to load local event filters as we already have with source plugins.
2. Add a new flag to load both types and set a deprecation date for the old --sources
3. A combination of points 1 & 2

Reasoning:
I personally prefer the second option, because have to flags each per type of plugin I feel it like overload the CLI interface. That would require to read the plugins from two different folders since the location is the only way to know what kind of plugin is. First option would be easier to implement and it would require less changes.