S4 classes should importClass external ancestors

[hadley]

No description provided.

[hadley]

(From Michael Lawrence)

If a class is included in a method signature, or is passed in "contains" to setClass(), and it is not defined within the package, Roxygen should search the dependent packages for a class of the same name and generate an importClassesFrom() directive. User should be able to override, just in case it picks the wrong package.

When a method is defined for a generic not defined in the package, search the dependencies and importFrom() it.