Add key-dependent attrsOf type

[prescientmoon]

It is sometimes useful for the type of attributes in attrsOf to depend on the current key. My internal implementation looks something like this

    dependentAttrsOf =
      name: mkType:
      let withErrorContext = addErrorContext "in ${name} value";
      in
      k.typedef' name
        (v:
        if ! lib.isAttrs v then
          typeError name v
        else
          withErrorContext
            (mkVerify
              (lib.mapAttrsToList (k: _: mkType k) v)
              v));

The mkVerify is an internal function of mine which looks similar to all' in your implementation, except it shows every error instead of the first one.