Fixed variable name in FindMagnumPlugins
modules: make FindMagnumPlugins.cmake actually working.

Broken since f3f2fc8b02d19e92402849e6a2b9b72404408d3b and never tested
since. Sigh. Thanks a lot to @wivlaro for the hint.
modules: set MAGNUMPLUGINS_INCLUDE_DIRS only on deprecated build.

Also mention that properly in the docs.
modules: updated FindMagnum.cmake.
ColladaImporter: added TODO.