feat(compartment-mapper): Collect unused module descriptors

[kriskowal]

What is the Problem Being Solved?

When building an archive, the compartment mapper currently collects unused compartment descriptors, but the increased prevalence of package exports has created a lot of unused module descriptors.

Description of the Design

We should mark and sweep these too, as we do with the compartment “retained” flag.

Security Considerations

None.

Scaling Considerations

Should reduce bundle size.

Test Plan

Compatibility Considerations

May frustrate hashes of existing applications when they upgrade bundle-source in their package lock. I hope we no longer support system-level agoric CLI since upgrading that will frustrate hashes without opt-in.

Upgrade Considerations

None.