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

[rolandwalker]

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.

Also add to_s methods to all sources and show resolved Cask source when
using --debug.

[deiga]

Did this also fix the cwd lookup for brew cask list?

[rolandwalker]

brew cask list <caskname> also does ignores files in the cwd (unless the named Cask is not present in a Tap).

The issue with file extensions also appears to have been fixed with these changes. I'm trying to decide when to cut a release: maybe today, certainly by Saturday.

Thanks for the report, and for the followups on these important details.