packages with symlinks are refetched on every build

[punchagan]

Expected Behavior

When running dune build in a project that uses package management, any dependencies that have symlinks (for example menhir) are re-downloaded since symlinks are not cached.

Actual Behavior

As a user, I would've expected that nothing gets downloaded on each build assuming there aren't any updates to packages.

Reproduction

#14429 added a repro cram test for this issue

[punchagan]

@Alizter and I discussed a potential fix for this, which he wrote down here

[Alizter]

The potential fix I discussed has a few downsides and it's important to be aware of them. Currently the download-and-extract-in-a-single-action rule never populates the dune shared cache with the downloaded tarball. This is good because tarballs can be quite fat and would inflate the shared cache a lot. The shared cache works better with many smaller files which the extraction step provides.

The issue with not caching the symlinks in the downloaded sources should probably be fixed by resolving those symlinks in a similar way to the directory symlinks in #13792. In fact, it should be pretty simple to do building on #13792.