This logic is all tightly coupled in the bin. This logic has been
notoriously impossible to test due to weird fs timing issues. By
extracting the actual logic we're now able to test it in isolation.

With this in place replacing Gaze (#636) becomes a viable option.
This PR not only massively increases our test coverage for the
watcher but also address a bunch of known edge cases i.e. orphaned
imports when a files is deleted.

Closes #1896
Fixes #1891