Allow directories as input

[juanrgon]

This PR allows for directories to be used as input to reorder-python-imports.

[asottile]

The intention is to run the tool through pre-commit which means every tool in the world doesn't have to reimplement:

• file matching (complicated, involves exclusion / inclusion / recursion / etc.)
• dry run / diff / etc.

I don't have an interest in supporting these complicated options in each and every tool, pre-commit run --all-files does a much better job than any tool could since it only will run against git tracked files (and the hook metadata configures what is included vs excluded). pre-commit run --all-files --show-diff-on-failure is the intended CI tool as well, which (as you'd expect) shows a diff.

See also:

• Recursive mode? pyupgrade#46
• Run on a file or directory pyupgrade#40
• Add check mode #45 (aside: reorder-python-imports gained some of these features before pre-commit had better support for this)

[juanrgon]

👍 I'll use my fork going forward