Improve handling of tarballs that don't consist of a single top-level directory

[edolstra]

Motivation

When unpacking tarballs, we now only strip the top-level path component if the tarball consists of a single directory or a single non-executable file.

Fixes #4785: top-level directories are no longer merged into one.

Fixes #10983: top-level non-directories are no longer discarded.

Closes #9053.

Context

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[roberth]

top-level non-directories are no longer discarded. Note that there still is a behavior change for tarballs that consist of a single executable file: to preserve the executable bit, these are returned as an executable file inside a directory.

I think this should also be the behavior for single non-executable files. Tarballs are expected to behave like directories, so we should probably always return a directory, even if it only has a single file.
Currently it's returning an empty directory - ie it's broken, so unless this is a regression, it seems we're free to pick the best behavior for this.

[edolstra]

I think this should also be the behavior for single non-executable files.

Yeah, it looks like even Nix 2.3 returned an empty directory so I'll change it to dereference directories only.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-07-29-nix-team-meeting-minutes-165/49970/1