source-bundle: don't include or build the "base image"

Typically a BuildStream project will contain a low level dependency that is a base image containing a shell and low-level utilities required to build the project.

A source-bundle creates a tarball that can be used to build the project in a pre-existing environment. As it stands, attempting to build the "base image" in a source bundle will attempt to install the utilities over the top of the pre-existing environment.

I think it would be desirable to have an option that enabled excluding a .bst and everything below it the the dependency hierarchy from a source bundle. This would create source bundles that could be significantly smaller and didn't attempt to re-install base image artifacts.