`detected_vcs_repo` attribute for Resource

[JonoYang]

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.

[pombredanne]

@JonoYang I wonder instead if we should not have a scanner function that handle the VCS detection?
And we may want to have a generic Resource attribute that states that a resource is "ignored" (similar to the "filtered" ?

With this we would populate a Codebase and would include the "ignored" resources. The default codebase walk would be to ignore ignored resource but that could be overriden by a scanner that could then also seen "hidden"/ignored resource and do its VCS detection? (NB: I am not convinced as this sounds a tad contrived)

[JonoYang]

@pombredanne I like the idea of including the ignored files in the Codebase, but not visiting them during a walk by default.