Use plugin model for targets

[ericwb]

Is your feature request related to a problem? Please describe.
If more target types are added, it's better to use a plugin type model than hard-code into CLI main.

Describe the solution you'd like
Two targets are supported currently, files and a GitHub repo. If PyPI source is also added as a target, then might as well create a plugin model for such.

Should use same extension model used for rules and parsers.

Describe alternatives you've considered
Continuing to hard code.

Additional context
n/a

Love this idea? Give it a 👍. We prioritize fulfilling features with the most 👍.