Remove code from `__init__.py` files

[JiriPavela]

As part of a preparation for adopting lazy imports and loading as described in #223, we should remove problematic code from __init__.py files that might otherwise break lazy loading and cause subpackages to be unexpectedly loaded when they are not actually used. Code in __init__.py files that does not affect the rest of Perun's logic (i.e., does not propagate outside the specific subpackage) may be kept as is, if it turns out too difficult to refactor or remove.