nar-accessor: use tree, fixes readDirectory missing children
nar-accessor: use tree, fixes readDirectory missing children

Previously, if a directory `foo` existed and a file `foo-` (where `-` is any character that is sorted before `/`), then  `readDirectory` would return an empty list.

To fix this, we now use a tree where we can just access the children of the node, and do not need to rely on sorting behavior to list the contents of a directory.

dtzWill/nix,vcunat/nix,ehmry/nix,dezgeg/nix,dezgeg/nix,dezgeg/nix,rycee/nix,layus/nix,peti/nix,vcunat/nix,pikajude/nix,shlevy/nix,rycee/nix,shlevy/nix,NixOS/nix,NixOS/nix,layus/nix,peti/nix,ehmry/nix,zimbatm/nix,layus/nix,ehmry/nix,NixOS/nix,vcunat/nix,zimbatm/nix,peti/nix,rycee/nix,peti/nix,rycee/nix,NixOS/nix,dezgeg/nix,dtzWill/nix,vcunat/nix,peti/nix,dtzWill/nix,rycee/nix,layus/nix,ehmry/nix,zimbatm/nix,layus/nix,shlevy/nix,shlevy/nix,NixOS/nix,pikajude/nix,zimbatm/nix,shlevy/nix,dtzWill/nix,dtzWill/nix,ehmry/nix,dezgeg/nix,ehmry/nix,pikajude/nix