cmCxxModuleMapper: Fix transitive requirements computation
Previously CMake may generate incomplete transitive requirements in
CMakeFiles/<target>.dir/CXXModules.json and therefore in module mapper
for compiler, when source files were listed in CMakeList.txt in a
certain order.
This commit fixes the problem by correctly tracking unfinished
transitive requirements computation of module units.
There have been a simple circular test case whose circular dependency
was reported by build system. Now with this correct implementation it's
reported by CMake generating module mappers.
Fixes: #25465 (closed)
Backport: release
Edited by Brad King