Performance spans for dynamically imported modules

### Problem Statement

It would be nice to have spans for when python modules are loaded dynamically.

This could be instrumented:
https://docs.python.org/3/library/importlib.html#importlib.import_module

(This would also be useful in Django apps when loading the URL configurations)

See also:
https://github.com/getsentry/sentry-python/issues/988#issuecomment-1005513677