The check for skipping a subdirectory was applied to Python subdirs,
not to C++ subdirs. This meant that a skipped module would notice
only in calamares_add_plugin() that it should be skipped. The **rest**
of the CMakeLists.txt in the to-be-skipped modules' subdirectory
was not skipped, and so a test might be added for a module that
was skipped. Depending on how the test consumes the code/module
under test, that might fail to link.

Example module is *initramfs*. Reported by yonggan.