This was lost after 2.32 while making the accessor lazy. We can restore the support
for it pretty easily. Also this is significant optimization for nix nar cat.
E.g. with a NAR of a linux repo this speeds up by ~3x:
Benchmark 1: nix nar cat /tmp/linux.nar README
Time (mean ± σ): 737.2 ms ± 5.6 ms [User: 298.1 ms, System: 435.7 ms]
Range (min … max): 728.6 ms … 746.9 ms 10 runs
Benchmark 2: build/src/nix/nix nar cat /tmp/linux.nar README
Time (mean ± σ): 253.5 ms ± 2.9 ms [User: 56.4 ms, System: 196.3 ms]
Range (min … max): 248.1 ms … 258.7 ms 12 runs
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/
```
