Before this commit, the `flake` option was typed with `types.unspecified`.
This type get's merged via [`mergeDefaultOption`](ebb592a04c/lib/options.nix (L119-L128)), which has a line
```nix
else if all isFunction list then x: mergeDefaultOption loc (map (f: f x) list)
```
`lib.isFunction` detects an attrs in the shape of `{__functor = ...}` as
a function and hence this line substitutes such attrs with a function
(f: f x).
If now, a flake input has a `__functor` as it's output, this will
coerce the once attrs to a function. This breaks a lot of things later
in the stack, for example a later `lib.filterAttrs seive <LAMBDA>` will
fail for obious reasons.
According to @infinisil, `types.unspecified` is due to deprecation. In
the meantime this PR provides a specific fix for the specific problem
discovered.
ecae25c3ef