Remove automatic __init__.py generation (set incompatible_default_to_explicit_init_py to true)

The automatic generation of `__init__.py` files has been a source of odd, surprising, and incorrect behavior. e.g. it makes directories importable that shouldn't be importable. This is because it's logic is extremely simple: create an init.py file in every parent directory for a path that ends with .py or .so.

It's also expensive: it requires flattening the runfiles at analysis time.

Some of the venv site package work incidentally side-steps this because the way it creates symlinks "hides" some of the paths from the regular set of runfiles.

Addressing this is a bit more complicated because the incompatible_default_to_explicit_init_py lives in bazel, but the usage is in rules_python.

In any case, I think the todo list is:

1. [x] Add a rules_python flag to replace incompatible_default_to_explicit_init_py
2. [ ] Disable the behavior by default
3. [ ] Remove the dead code paths

Users can use `--flag_alias` to map (1) to the bazel builtin flag.

This is the rules_python side of https://github.com/bazelbuild/bazel/issues/7386