add support for using Lmod's `depends_on` in generated modules

[boegel]

supported since Lmod 7.6, see http://lmod.readthedocs.io/en/latest/098_dependent_modules.html

[runiq]

Triage: Can this be closed?

[boegel]

Triage: Can this be closed?

Yes, supported since EasyBuild 3.6.0 (Apr 2018), see #2391