McuSupportPlugin: check dependencies for CMake vars

The McuSupport plugin includes a class called McuDependenciesKitAspect

It is used by the Kit Manager (not part of this plugin), to check that the paths to 3rd party dependencies found in the Environment Variables are existing.

This check is performed by the plugin when the user enters the paths, but once the kit is created, if the path becomes invalid (e.g. the user edits the kit information, or the component has been moved in the file system), this check has to be done by the Kit Manager.

The task here is to adapt this class to do the same check in the case the paths are in the CMake Variables (as opposed to the Environment Variables).