lmod: module files are written in a root folder named by target family (#13121)

fixes #13005

This commit fixes an issue with the name of the root directory for
module file hierarchies. Since #3206 the root folder was named after
the microarchitecture used for the spec, which is too specific and
not backward compatible for lmod hierarchies. Here we compute the
root folder name using the target family instead of the target name
itself and we add target information in the 'whatis' portion of the
module file.