Problem: default config is not really smart and seems easier to just
unlicense it, i. e. let everyone do whatever they want with it.
Also it does not have SPDX header (like any other file in this repo,
but that's another story).
Solution: add SPDX header stating that the Unlicense applies.
Make Serokell copyright holder of it, AFAIK @martoon is fine with
that (and I am not sure whether copyright holder matters for
unlicensed files at all).
Problem: for reference starting from `./` anchors are not checked.
Solution: lookup for such references in gathered repo info failed,
stripping `./` resolves the issue.
Problem: there are files which we want to ignore completely,
while others can be references but refs in it we do not want
to check.
Solution: have two different fields in config correspondingly.
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.
