`detected_vcs_repo` attribute for Resource

Currently, we ignore VCS repos when creating the Codebase (https://github.com/nexB/scancode-toolkit/blob/develop/src/scancode/resource.py#L427). It would be convenient to have an attribute on the Resource that says if a VCS repo was detected and what kind type of VCS is used. This is useful In the context of origin summary (such as #1588) as we would like to summarize resources at logical breakpoints. One such breakpoint would be the root of a VCS repo, as that is a package-like grouping useful to us.

Right now, there is no specific way to single out VCS directories out of the other types we ignore.