Hooks should be defined in SitePackage.lua, not in lmodrc.lua

[casparvl]

Defining hooks in lmodrc.lua works if the lmodrc.lua files are read early enough in the process of a module load. However, #490 (comment) shows that this is only the case under certain circumstances, and by no way a guarantee. We should probably move registration of the hooks to SitePackage.lua, as this is the officially document location for hooks https://lmod.readthedocs.io/en/latest/170_hooks.html?highlight=hook#hook-functions

[ocaisa]

The positive thing is we should still be able to do that in a way that allows sites to customise the hooks.

We can point to our SitePackage.lua by setting LMOD_PACKAGE_PATH=/path/to/the/Site/Directory, and this can leverage hooks from the equivalent location under host_injections. @casparvl noted that changing LMOD_PACKAGE_PATH does not seem to require reconfiguring Lmod, so we can still have dynamic behaviour.

[casparvl]

In fact, it's mostly a renaming exercise: our create_lmodrc.py (for now), only writes hooks. I.e. what it creates is actually a SitePackage.lua, even though we save it as lmodrc.lua :). If we just name it that, park it in $EESSI_SOFTWARE_PATH/.lmod (where we now also put the lmodrc.lua and set

export LMOD_PACKAGE_PATH="$EESSI_SOFTWARE_PATH/.lmod"

we're done (I think). We do need to probably make sure that LMOD_CONFIG_DIR does not get set, and/or that the existing lmodrc.lua files get removed from the repository.