Support subproject builds in their root folder

[ruippeixotog]

I'm exploring making PureConfig to use this plugin as we'd like to have more flexibility on cross-compiling. In this project we have the build of each subproject split between two build.sbt files - on the root build.sbt we specify plugins, subproject dependencies and settings common to all subprojects, while we have a per-subproject build.sbt on their root folder with settings specific to that project.

This setup doesn't seem to work with sbt-projectmatrix. The per-subproject build.sbt is not read, which I suppose happens because the base directory for the generated subprojects is not the same as before.

[ruippeixotog]

@eed3si9n do you have any workaround for this that doesn't involve moving everything to the same build.sbt file? It looks like addSbtFiles was removed in SBT 1.x and I see in https://github.com/sbt/sbt/issues/2514 that it was a problem before.