The nixpkgs-unstable channel's programs.sqlite was used to identify
packages producing exactly one binary, and these automatically added
to their package definitions wherever possible.
Since it is a somewhat common library, with support for multiple qt
versions, it is safer to put each version of it in libsForQt5 and
qt6Packages attribute sets. Also, it is cleaner to put the darwin
if-else inside the expression, in relation to https://github.com/NixOS/rfcs/pull/140 .
