AppDirs: Fix loading plugins

[ryonakano]

Resources.LIB already has the /usr (or /app if Flatpak) prefix in meson.build so the previous code tries to search for /usr/usr/lib/x86_64-linux-gnu or /app/app/lib and causes the critical messages displayed on the terminal when launching and plugins not shown in the preferences dialog.

Before

user@VirtualBox-17958d80:~/work/photos$ LANG=C io.elementary.photos 
** Message: 01:20:46.822: main.vala:347: Photos Photo Manager 8.0.0

(io.elementary.photos:67956): GLib-GIO-CRITICAL **: 01:20:46.835: g_file_get_child: assertion '!g_path_is_absolute (name)' failed

(io.elementary.photos:67956): GLib-GIO-CRITICAL **: 01:20:46.835: g_file_get_child: assertion 'G_IS_FILE (file)' failed

(io.elementary.photos:67956): GLib-GIO-CRITICAL **: 01:20:46.835: g_file_get_child: assertion 'G_IS_FILE (file)' failed

** (io.elementary.photos:67956): CRITICAL **: 01:20:46.836: plugins_search_for_plugins: assertion 'dir != NULL' failed
** Message: 01:20:46.836: main.vala:57: Verifying database ?
** Message: 01:20:46.841: VideoSupport.vala:403: interpreter state cookie not found; assuming all video thumbnails are out of date

(io.elementary.photos:67956): GLib-GObject-CRITICAL **: 01:20:46.884: value "-1" of type 'gint' is invalid or out of range for property 'position' of type 'gint'

After

user@VirtualBox-17958d80:~/work/photos$ LANG=C io.elementary.photos 
** Message: 01:21:07.239: main.vala:347: Photos Photo Manager 8.0.0
** Message: 01:21:07.254: main.vala:57: Verifying database ?
** Message: 01:21:07.260: VideoSupport.vala:403: interpreter state cookie not found; assuming all video thumbnails are out of date

(io.elementary.photos:68045): GLib-GObject-CRITICAL **: 01:21:07.289: value "-1" of type 'gint' is invalid or out of range for property 'position' of type 'gint'