* part of DSL 1.0 review
* `destination_path` was always a bit vague (it refers to
Cask-specific, version-specific location under
`/opt/homebrew-cask/Caskroom`)
* here renamed `staged_path` to match upcoming command verb
`brew cask stage`
* rename also intended to reduce confusion when we implement
copying as a configurable alternative to symlinking
* transitional `destination_path` methods to remain while
Casks are converted (this was documented as a part of the
DSL, and used by 39 Casks in main repo)
* unrelated variables containing "stage" recast for clarity
- brew cask list now displays casks without backing ruby files
- casks without a source are displayed as "caskname (!)"
- these casks can be uninstalled, with the caveat that it only removes
their files from the caskroom (doesn't run pkg uninstall or anything,
since there's no ruby file to define what to do)
