Ensure module files are generated in the build tree

The ExternalMoleculeMacros should ensure that the module data files are
generated in the build tree of the external module, rather than
polluting the VTK tree they are building against. This relies on the
vtkModuleAPI using CMAKE_MODULE_PATH for the original module files.

Change-Id: If134458e014d29f2cd432e2f9248b5c13b50c96b
Merge topic 'external-modules' into master

b5bc9853 Ensure module files are generated in the build tree