When one has several anchors with the same name, github disambiguates them automatically,
but referencesto such anchors stay fragile.
Now when such anchors are referenced then an verification error is dropped.
Also, no more wrong "anchor does not exist" error when a github-dedupicated anchor is
referenced (the link was valid, but since in our vision anchors couldn't be renamed
automatically, we treated those link as invalid).
For instance, reference to `issues` github page is a valid one despite there is no such
file physically in the repository.
Now user can specify list of "existing" virtual files in config.
