Fix#11897
As described in the issue, this makes for a simpler and much more
intuitive notion of a realisation key. This is better for pedagogy, and
interoperability between more tools.
The way the issue was written was that we would switch to only having
shallow realisations first, and then do this. But going to only shallow
realisations is more complex change, and it turns out we weren't even
testing for the benefits that derivation hashes (modulo FODs) provided
in the deep realisation case, so I now just want to do this first.
Doing this gets the binary cache data structures in order, which will
unblock the Hydra fixed-output-derivation tracking work. I don't want to
delay that work while I figure out the changes needed for
shallow-realisations only.
This reverts commit bab1cda0e6.
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/
```
