Make it possible to use unmodified upstream briar

I know we have this solution for making briar work as a submodule by applying a patch on the upstream version, but it bugged me we need to do this (while trying to make the same thing work for briar-swing).

I now found a solution that makes it actually possible to use briar unmodified.

Basically we can leave the Gradle modules' names unmodified and instead configure the path where to find those modules.