BUG: Fix vtk_module_config for repeated calls

Fix use of _${ns}_${mod}_USED settings to block reporting a module more
than once in a single vtk_module_config call.  Do not leak the
_${ns}_${mod}_USED settings outside a single call to vtk_module_config
so that a future call to the macro with the same ${ns} will use a fresh
set of modules.

Reported-by: Andrew Maclean <andrew.amaclean@gmail.com>
Reported-by: Marcus D. Hanwell <marcus.hanwell@kitware.com>
Change-Id: Ib37717f5724ad94e20c14165886a0d023fcecd9c
Merge topic 'module-api' into master

f915a546 BUG: Fix vtk_module_config for repeated calls