Summary

For context, we have three extraction paths:

• untar (async) - used for any .tar.gz, local or remote.
• unzip (async) - used to unzip remote wheels, or local or remote source distributions.
• unzip (sync) - used to untar locally-available wheels into the cache.

We use three different crates for these:

• tokio-tar
• async-zip
• zip-rs

These all seem to have different support for symlinks:

• tokio-tar tries to create a symlink (which works fine on Unix but errors on Windows, since we typically don't have elevated permissions).
• async-zip seems to write the target contents directly to the file (which is what we want).
• zip-rs apparently writes the name of the target to the file (which isn't what we want).

Thankfully, symlinks are not allowed in wheels (pypa/pip#5919, https://discuss.python.org/t/symbolic-links-in-wheels/1945), so we can ignore zip-rs.

For tokio-tar, we now skip (and warn) if we see a symlink on Windows. We could do what pip does, and recursively copy, but it's difficult because we don't have Seek on the file. (Alternatively, we could use hard links and junctions, though those also might need to exist already.) Let's see how far this gets us.

(We also no longer attempt to set permissions on symlinks on Unix, which caused another failure.)

Closes #1858.