postgresql: libpq-only lib output

[szlend]

Description of changes

This change makes it so only libpq.* is moved to the lib output.

Advantages:

• Smaller postgresql.lib
• Potentially less workarounds patching $lib->$out. In theory we should only have to do this for libpq now.

Disadvantages:

• It's not clear what LIBDIR = in pg_config should point to since there are now two /libs. It could potentially break some builds that rely on pg_config.
• Potentially a breaking change for postgresql.lib users.

Previous discussions: #61580 #234470

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[szlend]

This is only a draft. Just curious about the initial thoughts on this approach from maintainers who have more experience dealing with postgres. cc @thoughtpolice @marsam

I tested this with a sample project using libpq through postgresql.lib and it seems to work fine. But I'm not familiar with the intricacies of the postgres server and all its extensions.

[szlend]

Closing in favor of #294504