🐾 look in ./plugins for finding fdroid plugins

With this change fdroidserver will also check the local ./plugins directory when looking for plugins. It's a convenience feature and allows repo maintainers to easily add and share repository specific fdroidserver sub-commands.

E.g. I'm using the .plugins for collaborating on building a repository of .IPA files. This repository requires some additional tooling for handling apple apps. (With this change I can implement all that tooling as plugins. This way it's easy to move to plugins into a separate package or upstream them into fdroidserver should in case some of these plugins catch on.)