Continue moving personal packages from overlays

Continue moving out of the overlay model for everything and into running
sutff directly out of the packages. This should give a cleaner
separation of the things that I don't need to maintain separately

pkgs/upgrade-pg-cluster.nix

@@ -0,0 +1,36 @@
+{
+  writeShellApplication,
+  postgresql_15,
+  postgresql_16,
+  ...
+}:
+
+let
+  newPostgres = postgresql_16;
+  oldPostgres = postgresql_15;
+in
+writeShellApplication {
+  name = "upgrade-pg-cluster";
+  # Only the new one should be in scope, because that is the
+  # one that we will be referencing directly with the sudo command
+  # down farther.
+  runtimeInputs = [ newPostgres ];
+  text = ''
+    systemctl stop postgresql
+
+    export NEWDATA="/var/lib/postgresql/${newPostgres.psqlSchema}"
+    export NEWBIN="${newPostgres}/bin"
+
+    export OLDDATA="/var/lib/postgresql/${oldPostgres.psqlSchema}"
+    export OLDBIN="${oldPostgres}/bin"
+
+    install -d -m 0700 -o postgres -g postgres "$NEWDATA"
+    cd "$NEWDATA"
+    sudo -u postgres "$NEWBIN/initdb" -D "$NEWDATA"
+
+    sudo -u postgres "$NEWBIN/pg_upgrade" \
+        --old-datadir "$OLDDATA" --new-datadir "$NEWDATA" \
+        --old-bindir "$OLDBIN" --new-bindir "$NEWBIN" \
+        "$@"
+  '';
+}