remove niv residue

```fish
for d in {.,**/*}
    if [ ! -d $d ]; continue; end

    set from ./nix/sources.nix
    set to   ./npins

    set from (realpath -s $from --relative-to $d)
    set to   (realpath -s $to   --relative-to $d)

    echo "($d, $from, $to)"

    for f in $d/*
        if [ ! -f $f ]; continue; end
        sed -i "s=$from=$to=" $f
    end
end
```

nix/packages/default.nix

@@ -1,4 +1,4 @@
-{sources ? import ../sources.nix}: let
+{sources ? import ../../npins}: let
   scopeOverlay = overlay: final: prev: {export = prev.export or {} // overlay final prev;};
 in
   (