Set `top-level` to default to true

[kstrafe]

I don't see the point in defaulting to seeing the results that are not --top-level. For most use-cases I've had I only wanted to know which derivations provided some file, not what subderivations provided some file.

What benefit is it to me to see:
(zulip.out) 14,712 x /nix/store/ymihk5dxg7m5jkfmgk7vmbxw5gy6hr5i-python3-3.9.13/bin/python3.9

for instance when searching for bin/python?

I'm not arguing to remove it (maybe someone wants to find out what uses some derivation), but I am arguing that this is not the default/common use-case.

[Dawid Dziurla]

Should be done in #243