Cache wide operations

It would be nice to be able to straightforwardly:
- download all the missing or new versions of the cached artifacts (and their transitive dependencies),
- download all the missing docs and/or sources of the cached artifacts.

That should allow for a seamless offline mode, for both compiling and from an IDE (with sources/docs artifacts).

Start by tracing the cache and artifact-fetching entry points related to cached artifacts, transitive dependencies, documentation, and sources. Define the scope and behavior for downloading missing or new items, then verify that compilation and IDE use can proceed offline with the required artifacts.

Written by the indexing model from the issue text.