Restore `builtins.pathExists` behavior on broken symlinks
(cherry picked from commit d53c8901ef7f2033855dd99063522e3d56a19dab)
===
note that this variant differs markedly from the source commit because
we haven't endured quite as much lazy trees.
Change-Id: I0facf282f21fe0db4134be5c65a8368c1b3a06fc
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/
```
(cherry picked from commit 68c81c7375)
eldritch horrors
