allow string imports outside of packages

[carloshorn]

Problem description

Currently, string imports are not possible outside of packages.

Minimal reproducible example

The following code snippet executed from a script or Jupyter notebook cell raises an AttributeError: 'NoneType' object has no attribute '__package__'

from dependency_injector import providers
deque_factory = providers.Factory('collections.deque')

The trace back reveals that the exception is thrown in src/dependency_injector/providers.pyx in dependency_injector.providers._resolve_calling_package_name().

Proposal for solution

In case the module in dependency_injector.providers._resolve_calling_package_name is None, the package should also be None. The subsequent call to importlib.import_module will handle the situation properly.