pidgin/pidgin

Convert the gplugin subproject to use the auto fallback stuff of dependency.

Testing Done:
Verified the the purples gir's loaded and were able to call into gplugin under python, when built as a subproject and against a system installed version.

Reviewed at https://reviews.imfreedom.org/r/937/
---
id: im.pidgin.Finch3
runtime: org.gnome.Platform
runtime-version: '3.36'
sdk: org.gnome.Sdk
command: finch3
finish-args:
- --share=ipc
- --socket=x11
- --share=network
- --talk-name=org.freedesktop.Notifications
- --system-talk-name=org.freedesktop.Avahi
modules:
- packaging/flatpak/libpurple-common.yml
- packaging/flatpak/libgnt.yml
- name: finch3
buildsystem: meson
config-opts:
- -Dgtkui=false
- -Ddoc=false
sources:
- type: dir
path: '.'