DSL: add stanza `stage_only` (new `caskroom_only`)

`caskroom_only` was never documented.  Its original purpose was
obsoleted in #4865, and its use has been recently been reduced to
two Casks.

This PR
* continues the rationalization of naming by changing `caskroom_only`
  to `stage_only`. "stage" is the verb for "make a copy under the
  caskroom directory"
* documents `stage_only`
* adds tests for `stage_only`
* validates the argument to `stage_only`
* gives sensible output in `brew cask info` for `stage_only` Casks
* enforces that `stage_only` cannot coexist with any activatable
  artifacts

`caskroom_only` is still supported for backward compatibility,
but should be removed before 0.50.0.