nix: Build only libpq instead of full postgresql package

nix/overlays/default.nix

@@ -4,7 +4,7 @@
   gitignore = import ./gitignore.nix;
   haskell-packages = import ./haskell-packages.nix;
   postgis = import ./postgis.nix;
-  postgresql-default = import ./postgresql-default.nix;
+  postgresql-libpq = import ./postgresql-libpq.nix;
   postgresql-legacy = import ./postgresql-legacy.nix;
   postgresql-future = import ./postgresql-future.nix;
   slocat = import ./slocat.nix;

nix/overlays/haskell-packages.nix

@@ -1,4 +1,4 @@
-{ compiler, extraOverrides ? (final: prev: { }) }:
+{ compiler }:
 
 self: super:
 let
@@ -41,11 +41,14 @@ let
       # Marked as broken (?)
       fuzzyset = lib.markUnbroken prev.fuzzyset;
 
-      postgresql-libpq = lib.dontCheck prev.postgresql-libpq_0_10_0_0;
+      postgresql-libpq = lib.dontCheck
+        (prev.postgresql-libpq_0_10_0_0.override {
+          postgresql = super.libpq;
+        });
 
       hasql-pool = lib.dontCheck prev.hasql-pool_0_10;
 
-    } // extraOverrides final prev;
+    };
 in
 {
   haskell =

nix/overlays/postgresql-default.nix

@@ -1,5 +0,0 @@
-self: super:
-# Overlay that sets the default version of PostgreSQL.
-{
-  postgresql = super.postgresql_15;
-}

nix/overlays/postgresql-libpq.nix

@@ -0,0 +1,6 @@
+self: super:
+{
+  libpq = super.callPackage ../libpq.nix {
+    postgresql = super.postgresql_15;
+  };
+}