Don't prefer Cask files in the cwd

Fixes #4641.  Cask files in the cwd are still permitted, but
at the very lowest priority, below Tapped Cask names.
show resolved Cask source file or URI w/ --debug

references #4641
doc: improve Cask-source-resolution debug message
Merge pull request #4658 from rolandwalker/demote_implicit_path_source

Demote implicit Cask path sources (i.e. files in current working directory)