postgresql: cleanup postgis (#54396)

postgis: cleanup

Another part of https://github.com/NixOS/nixpkgs/pull/38698, though I did cleanup even more.
Moving docs to separate output should save another 30MB.

I did pin poppler to 0.61 just to be sure GDAL doesn't break again next
time poppler changes internal APIs.

pkgs/servers/sql/postgresql/packages.nix

@@ -1,5 +1,12 @@
 self: super: {
 
+    postgis = super.callPackage ./ext/postgis.nix {
+        gdal = self.gdal.override {
+            postgresql = self.postgresql;
+            poppler = self.poppler_0_61;
+        };
+    };
+
     pg_repack = super.callPackage ./ext/pg_repack.nix { };
 
     pg_similarity = super.callPackage ./ext/pg_similarity.nix { };