Allow running codemods without configuring in YAML

[akx]

Summary

This enables codemodding things by just plonking a CodemodCommand class into any old importable module and running python -m libcst.tool codemod -x some_module.SomeClass ...

[povilasb]

👍

[zsol]

I'm in favor of this functionality, but

1. let's make it explicit behind a command line flag instead of an implicit fallthrough behavior:

python -m libcst.tool codemod --external-command some_module.SomeClass

(feel free to come up with a better name 😄 )
2. let's add tests for it

[akx]

@zsol How about a flag to allow the fallback behavior?

python -m libcst.tool codemod -x some_module.SomeClass

(-x for eXternal)

[zsol]

What's the benefit of that over the explicit behavior I mentioned above?

[akx]

@zsol Yeah, good point – I was about to edit my comment to a "never mind" because I realized that was silly after all, but you got there first 😁

Anyway, um... are there tests for the CLI at all? test_tool.py seems to have unrelated things in it... (EDIT: no, there don't seem to be thoroguh tests for the code paths in the CLI that do things.)

[zsol]

Looks great, thanks!

[zsol]

Nit: this else block is not needed, we can just dedent the entire block since we always return in the if branch

[akx]

Um... no, we don't return from the if args.external branch?

[zsol]

Oh you're absolutely right 😂

[zsol]

Thanks!