class Inspec::SourceReaderRegistry

always receive a fetcher.
Pre-checking of target resolution. Make sure that SourceReader plugins

def resolve(target) 

def resolve(target)
  return nil if target.nil?
  unless target.is_a? Inspec::Plugins::Fetcher
    fail "SourceReader cannot resolve targets that aren't Fetchers: #{target.class}"
  end
  super(target)
end