Small improvements to ModuleLoadEnvironment

[Flamefire]

I just checked #4653 and found a few possible improvements:

1. Using an "unknown" state and requiring callers to update that is error-prone. Comparing self.install_lib_symlink == LibSymlink.LIB64_TO_LIB might silently fail if it is forgotten. Use a property with auto-init instead
2. Isn't def __init__(... var_type=ModEnvVarType.PATH_WITH_FILES...) clearer in what the default is instead of using and checking for None?
3. for attr in self.__dict__: yield attr, getattr(self, attr) can be made simpler and faster by using yield from self.__dict__.items() or just returning the latter
4. environ can use a dict-comprehension for brevity and speed to avoid repeated update calls

Do those make sense? @lexming @boegel

[boegel]

lgtm

[lexming]

@Flamefire yeah, those make perfect sense, thanks for the PR!