Add module file to build on top of EESSI

[ocaisa]

If we allow a default approach to where extensions to EESSI are stored ${EESSI_SOFTWARE_PATH/versions/host_injections} (as is done in #172) then there is nothing to stop us shipping a module that one can load to extend EESSI with all the required settings. The module only needs to set:

export EASYBUILD_IGNORE_OSDEPS=1
export EASYBUILD_SYSROOT=${EPREFIX}
export EASYBUILD_RPATH=1
export EASYBUILD_FILTER_ENV_VARS=LD_LIBRARY_PATH
export EASYBUILD_FILTER_DEPS=$DEPS_TO_FILTER (calculated)
export EASYBUILD_MODULE_EXTENSIONS=1
export EASYBUILD_PREFIX=${EESSI_SOFTWARE_PATH/versions/host_injections}

It would also need to check that we have rights to create/write to $EASYBUILD_PREFIX