[cxxmodules] Fix absolute paths for v7 headers in modulemap

Our CMake code for making path to headers relative is broken
for the v7 headers (as they don't have the 'CURRENT_SOURCE/inc'
prefix we check for). This caused that we have absolute paths for
those headers in the modulemap we ship with ROOT. As the paths
are only valid on the same system in the specific build folder,
they prevent anyone from successfully building any ROOT module
with this modulemap.

This patch adds the additional check for v7 headers and also
adds a fallback where we always remove the current source dir
from the paths (which is necessary for all the tests which
don't follow any directory structure).

Also adds a sanity check that should prevent this error in the
future by checking that paths are not absolute before we write
them to the modulemap.