Support for `-SNAPSHOT` versions that do not cache the package locally

I don't know exactly how caching works on `pkl`.
But I assume it is just basic support yet.

Once a package was requested, it will be stored in the cache directory.
If requested again it will be used from the cache directory instead from the package server.

Coming from the JVM-World I think it would be quite nice to support `-SNAPSHOT` versions in the same way Gradle and Maven (I guess) supports it.

Being said, if requested a -SNAPSHOT version it will "always" check the package server to find out if there is a new version of that SNAPSHOT version.

How this works behind the scenes (in the Gradle world)?
Each time you create/publish a SNAPSHOT version the tool will add a timestamp to the package (to distinguish between multiple SNAPSHOT versions).
If requested, the "latest" (timestamp) SNAPSHOT version will be downloaded.

However, how pkl support could look differently 🙃

The issue names no files, tests, or entry points. Start by locating package resolution and cache handling in the repository, then clarify how snapshot versions should be identified, refreshed, and verified against the package server; done means the agreed snapshot behavior is covered by tests.

Written by the indexing model from the issue text.