This commit

(1) refactores the handling of optional dependencies in module.cmake

and

(2) generalizes the approach implemented in 34445909 (ENH: Add
remote modules if they are loaded) to work with VTK >= 8.1. This means
that optional dependencies will be found in scenarios like:
* WikiExamples standalone, all its dependencies built-in module or Remote modules
* WikiExamples remote, all its dependencies built-in module or Remote modules
* WikiExamples remote, some of its dependencies as standalone, remote or built-in modules


Former-commit-id: e9e339ae