chore(null): fix yarn null:find on windows

[dbjorge]

Description of changes

The primary impact/motivation here was to fix an issue where yarn null:find wasn't outputting all the files it was supposed to (as it turns out, this was windows-specific). The fix for this is on line 39 of import-finder.js (different parts of the tool weren't in agreement about normalizing slashes, which was breaking some of the filtering logic).

As part of fixing/debugging this, I did a fair amount of refactoring/cleanup to null:find. I didn't separate out the cleanup into a separate PR like I normally would because this is a short-lived dev-only tool that doesn't require as thorough review as most of the repo's code.

Pull request checklist

• Addresses an existing issue: related to Codebase should use strict null checks #2869
• Ran yarn fastpass
• [n/a] Added/updated relevant unit test(s) (and ran yarn test)
• [n/a] Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.
• [n/a] (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS