driver: Fix interaction of -Wunused-packages and reexported-modules

Spurious warnings were previously emitted if an import came from a reexport due to how -Wunused-packages were implemented. Removing the dependency would cause compilation to fail.

The fix is to reimplement the warning a bit more directly, by searching for which package each import comes from using the normal module finding functions rather than consulting the EPS. This has the advantage that the check could be performed at any time after downsweep rather than also relying on a populated EPS.

Fixes #19518 (closed) and #19777 (closed)