Proxied apt repo does not merge cached packages into metadata

When using a proxy apt repository packages are cached in the proxy, however once versions are removed from the host repository those versions are also no longer downloadable through the proxy repository even though the packages are available in the cache.

This most likely is caused by not enhancing (merging) the host repository metadata with metadata related to the cached packages not available in the host repository anymore.

Without merging cached packages back into the host repository metadata the proxy repository is almost unusable for reproducible package distribution setups.

Note: the snapshot feature does not solve the problem completely as snapshots are always done based on current host repository metadata, hence subsequent snapshots lose older versions not present on the host repository anymore.