Solve lazy imports to speed up Perun

[JiriPavela]

With demandimport being no longer supported in Python 3.12 (#121), it is the ideal opportunity to find an alternative lazy import library (or write our own) so that we can finally speed up Perun startup time. Some more discussion will likely be needed to select the most suitable approach for lazy imports.

Some suggestions:

• Local imports where applicable (note that pylint warns about this)
• Importlib LazyLoader: https://docs.python.org/3/library/importlib.html#importlib.util.LazyLoader
• 3rd party package: https://github.com/telekom/lazy-imports