Fine-grained CDS.

The current implementation will dump ALL packages imported after the `cds` module gets loaded, which might cause unnecessary footprint during load. We need a mechanism to verify if this is the case, and if so, find a way to get rid of unused pacakges.

The issue does not name any files, tests, or entry points. Start by locating the CDS module and the code that dumps imported packages, then determine how to verify whether all post-load imports are included and how unused packages could be excluded. Done means the mechanism is defined and its footprint impact can be demonstrated.

Written by the indexing model from the issue text.