Right now nixpkgs only has postgresql, which contains much more than libpq.

By offering a libpq package like other distros, the closure size of many packages could be reduced significantly.

Even more so because postgresql currently depends on systemd. And that also means that no application or library using libpq can be built against musl, as systemd relies on glibc-only features and thus does not build with musl.

Tasks:

• Make postgresql -> systemd dependency configurable
• Add libpq package containing only libpq
• Change Haskell packages so that they depend on libpq instead of all of postgresql
• Change other packages that can benefit from this (help wanted for the other programming languages!)