Deprecate adding a non-existing path to $MODULEPATH

[Flamefire]

The module.use function is not actually supposed to create a directory, but we require that the directories of $MODULEPATH exist in curr_module_paths

Hence deprecate adding a path which does not exist

As a side effect we need to explicitly disallow the empty path '', as that also does not exist. EnvironmentModules ignores such paths, LMod does create an empty entry in $MODULEPATH but doesn't really use it. We also filter it in curr_module_paths so it makes sense to fully disallow adding it.

During the tests on CI another bug was discovered: The module tools (at least Lmod) strip trailing slashes before adding a path to $MODULEPATH. However we sometimes do add_module_path(os.path.join(some_prefix, modpath)) where modpath could be empty which leads to a trailing slash and add_module_path (and remove_module_path) not finding the path in curr_module_paths (which do not contain the trailing slashes) This means that paths wrongly were retained in $MODULEPATH or unnecessarily added again.

I added a test to catch this issue and fixed it.

I also had to add checks before adding Hierarchical MNS paths, such as Core if they exist (already). Not adding non-existing ones is fine as far as I can tell and even leads to more speed by avoiding module calls, cache invalidation and work for the modules tool (repeatably checking a non-existing folder)

[akesandgren]

LGTM
Still more places where we should do os.pathsep.join and os.path.join but let's leave that for another cleanup PR someday.

[akesandgren]

Going in, thanks @Flamefire!