This change lets projects like ParaView, that use VTK as a
submodule, reuse VTK's CMake code for their own
modules which are maintained outside of VTK's repository.

To specify additional directories to search in, call either of
the two new macros in vtkModuleAPI to build up the search path
before adding VTK as a cmake subdirectory.
For example:
include (vtkModuleAPI)
vtk_module_src_glob_path_add(
  "${ParaView_SOURCE_DIR}/ParaViewCore/modular"
  "${ParaView_BINARY_DIR}"
)
vtk_module_src_path_add(
  "${ParaView_SOURCE_DIR/MyModule"
  "${ParaView_BINARY_DIR/MyModule"
)
add_subdirectory(VTK)

Change-Id: Ic738ad289e330a20215c7a2e644ce52cf1156413