When --unpack was used the nix would add the current directory to the
nix store instead of the content of unpacked.
The reason for this is that std::distance already consumes the iterator.
To fix this we re-instantiate the directory iterator in case the
directory only contains a single entry.
Having a narHash doesn't mean that we have the other attributes
returned by the fetcher (such as lastModified or rev). For instance,
$ nix flake metadata github:NixOS/patchelf/7c2f768bf9601268a4e71c2ebe91e2011918a70f
Last modified: 2024-01-15 10:51:22
but
$ nix flake metadata github:NixOS/patchelf/7c2f768bf9601268a4e71c2ebe91e2011918a70f?narHash=sha256-PPXqKY2hJng4DBVE0I4xshv/vGLUskL7jl53roB8UdU%3D
(does not print a "Last modified")
The latter only happens if the store path already exists or is
substitutable, which made this impure behaviour unpredictable.
Fixes#10601.
I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests
- Concepts is harder to understand, the documentation makes a good
unit vs functional vs integration distinction, but when the
integration tests are just two subdirs within `tests/` this is not
clear.
- Source filtering in the `flake.nix` is more complex. We need to
filter out some of the dirs from `tests/`, rather than simply pick
the dirs we want and take all of them. This is a good sign the
structure of what we are trying to do is not matching the structure
of the files.
With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests
functional/
installer/
nixos/
```
