postgresql: refactor to move packages.nix to ext/default.nix (1/2)

This commit is split up into two commits to allow git to detect renames,
make rebasing easier and allow a working entry in .git-blame-ignore-revs.

To allow bisecting we allow evaluation on every commit by moving the extensions
into ext/ext/ first and back to ext/ with the next commit.

pkgs/servers/sql/postgresql/default.nix

···
         };
         newSelf = self // scope;
         newSuper = { callPackage = newScope (scope // this.pkgs); };
-      in import ./packages.nix newSelf newSuper;
+      in import ./ext newSelf newSuper;
 
       withPackages = postgresqlWithPackages {
                        inherit makeWrapper buildEnv;