generate-programs-index finds wrong attrPath for "dig" binary

[delroth]

sqlite> select * from Programs where name = "dig";
dig|x86_64-linux|bind

Which is technically correct: dig is part of the bind derivation, but not in its default output. Instead, it's part of the dnsutils output which does not get installed by a simple nix-shell -p bind (it needs either nix-shell -p bind.dnsutils or nix-shell -p dnsutils since there is an alias in top-level).

Something somewhere in generate-programs-index should probably be more output-aware, but I lack the ability to easily test changes so this is just an issue report and not a PR :-)