This commit makes the following changes:

- Introduces a new `load_instrumentor(EntryPoint) -> None:` with a
default implementation method to the `BaseDistro` class.
- The default implementation loads the insrumentor from the provided
entry point and calls applies it without any arguments. (same as before)
- sitecustomize now calls Distro's `load_instrumentor` method to load
and activate an instrumentor instead of doing it directly.
- Added a new `DefaultDistro` implementation which is used if not distro
is found by entry points.