use "staged" instead of "installed" for Caskroom files in messages

Following up on #6783.  This is imperfect and far from complete, as the "installed" terminology is used extensively in source, comments, messages, and documentation.

The intention is to clarify that there are two phases to `install`: staging and activation.

* Staging is the copying of the distribution to the Caskroom.  Clarifying the docs is a step toward exposing a separate `brew cask stage` verb.
* Activation is the creation of symlinks against the staged distribution.  Here we have an interest in clarifying the docs so that future versions can use copying, hardlinking, or symlinking for the activation phase.  A side benefit of exposing a `brew cask activate` verb would be enabling certain multi-user scenarios.

Homebrew uses `brew link` for the activation phase.  Since we specifically would like to support methods other than linking, we must diverge from the Homebrew interface and use a verb other than `link`.

Exposing these new verbs does not mean that `brew cask install` would need to change; `install` would continue to invoke all needed phases, up to and including "activation".