Support local key assets for package-repositories

### What needs to get done

The current implementation of package-repositories does not support local key assets (the local dir is always /dev/null).

### Why it needs to get done

Snapcraft has support for keys in `snap/keys`

Locate the package-repositories implementation and trace where the local directory is set to /dev/null. Compare that behavior with Snapcraft's snap/keys support, then identify or add focused tests for local key assets. Done means package-repositories can use keys from snap/keys rather than always using /dev/null.

Written by the indexing model from the issue text.