package resolve does not save packages in cache

Right now resolving packages doesn't store packages in the cache.

This is in some situations problematic because "fully offline use" is not possible with the current implementation.
Except of the fact that it is also desired to cache packages to avoid unnecessary API calls.

Found out via [this discussion](https://github.com/apple/pkl/discussions/346).
See also this comment: https://github.com/apple/pkl/discussions/346#discussioncomment-8921705

Start by tracing the package-resolution path and the existing cache handling; the issue does not name files, tests, or entry points. Reproduce resolution with and without network access, then verify that resolved packages are cached, can support fully offline use, and do not trigger unnecessary API calls.

Written by the indexing model from the issue text.