fix(system_python): write import paths to generated file instead of using PYTHONPATH (#3242)

This changes the system_python bootstrap to use a 2-stage process like
the script
bootstrap does. Among other things, this means the import paths are
written to
a generated file (`bazel_site_init.py`, same as boostrap=script) and
sys.path
setup is performed by the Python code in stage 2.

Since the PYTHONPATH environment variable isn't used, this fixes the
problem on
Windows where the value is too long.

This also better unifies the system_python and script based bootstraps
because the
same stage 2 code and bazel_site_init code is used.

Along the way, several other improvements:

* Fixes path ordering for system_python. The order now matches venv
ordering
  (stdlib, binary paths, runtime site packages).
* Makes the venv-based solution work when the site module is disabled
(`-S`).
* Makes `interpreter_args` attribute and
`RULES_PYTHON_ADDITIONAL_INTERPRETER_ARGS` env var work with
system_python.
* Makes `main_module` work with system_python.
* Progress towards a supportable non-shell based bootstrap (a user
requested
this because their environment doesn't install any shells as a security
  precaution).

Fixes https://github.com/bazel-contrib/rules_python/issues/2652