Add support for multiple projects directories

rbm currently looks for projects definitions in the projects directory. If we want to re-use some projects from a different repository, we need to add that repository as a git submodule, and add symlinks from the projects directory to the components we want to re-use.

To make that easier, I think rbm could have an option to list multiple projects directories which would remove the need to add symlinks.

This would help for splitting tor-browser-build into different parts (tor-browser-build#40350).