warning: Using 'builtins.derivation' to create a derivation named 'devshell-env.bash' that references the store path '/nix/store/...-source' without a proper context. The resulting derivation will not have a correct store reference, so this is unreliable and may stop working in the future.

Automatically orders the env vars by dependencies based on the values
and what is in the `${}` parts. This does require every eval-like str
to use the full `${}` syntax, and not `$`.

Probably could get away with just parsing for `$[A-z0-9_]+` but this
is safer.

Allows a much simpler syntax for simple variable settings. These are
equivalent:

```
env = {A = {value = 1;};};
env = {A = 1;};
```

And there is also the null value shortcut to unset a variable.

```
env = {U = {unset = true;};};
env = {U = null;};
```

:(

Add a `package` as a value using `VARNAME.value = package`.