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/
syntax: glob
.*.swp
.DS_Store
.flatpak-builder
*.pyc
*~
\#*\#
subprojects/*/
syntax: regexp
^build(.+)?\/
win32-install-dir(\.release)?
po\/pidgin\.pot$