Ensure Makefile notices changes to our other repositories

[kzar]

The Makefile is set up to notice changes to the code, rebuilding as
necessary. Let's also have it watch for changes to our other
repositories (under node_modules/@duckduckgo/) and repositories that
have been combined with this one (under packages/).

Reviewer: @sammacbeth

Steps to test this PR:

1. Run make dev browser=chrome type=dev twice.
2. It should say "make: Nothing to be done for 'dev'."
3. Edit a file under packages/privacy-grade or node_modules/@duckduckgo/*
4. Run make dev browser=chrome type=dev and ensure the bundles are rebuilt.

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications