BUG: Fix itk_module_config for repeated calls

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

Change-Id: I422e415f45db877512a0dcd3c588d9f74d32f515
Merge topic 'module-api'

ee321c2a BUG: Fix itk_module_config for repeated calls