Reset VTK_MODULES_REQUESTED to all modules with multiple calls.

This generates more expected behaviour when

  find_package(VTK COMPONENTS X Y Z)

followed by

  find_package(VTK)

is called.  When no components are specified in subsequent calls, reset
VTK_MODULES_REQUESTED to all modules enabled.

Change-Id: I9264ed61614a7c221e7fcc354bd67afd437e258e