hooks: add moveSystemdUserUnitsHook

This hook moves systemd user service file from `lib/systemd/user` to
`share/systemd/user`. This is to allow systemd to find the user
services when installed into a user profile. The `lib/systemd/user`
path does not work since `lib` is not in `XDG_DATA_DIRS`.

pkgs/build-support/setup-hooks/move-systemd-user-units.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+
+# This setup hook, for each output, moves everything in
+# $output/lib/systemd/user to $output/share/systemd/user, and replaces
+# $output/lib/systemd/user with a symlink to
+# $output/share/systemd/user.
+
+fixupOutputHooks+=(_moveSystemdUserUnits)
+
+_moveSystemdUserUnits() {
+    if [ "${dontMoveSystemdUserUnits:-0}" = 1 ]; then return; fi
+    if [ ! -e "${prefix:?}/lib/systemd/user" ]; then return; fi
+    local source="$prefix/lib/systemd/user"
+    local target="$prefix/share/systemd/user"
+    echo "moving $source/* to $target"
+    mkdir -p "$target"
+    (
+      shopt -s dotglob
+      for i in "$source"/*; do
+          mv "$i" "$target"
+      done
+    )
+    rmdir "$source"
+    ln -s "$target" "$source"
+}