further cleanup/refactor of link artifacts

Following up on #2874, we should no longer need `linked_path`,
but instead simply refer to `target`.

This also fixes a bug in which some links were not removed
during `uninstall`.  Example:
  binary 'oclint-0.7-x86_64-apple-darwin-10/lib/oclint', :target => '/usr/local/lib/oclint'