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

[rickeylev]

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 #2652

[aignas]

Gave a quick look at this and I think the change is really exciting. I'll do a thorough review once it is ready for review.

[rickeylev]

fyi

@mering -- non-shell based bootstrap (still needs a system python, so not our coveted native launcher, but doesn't require you to have a shell installed)

@jacky8hyf -- should make bootstrap=script work with site disabled (-S)

[rickeylev]

Ok, ready for review

[aignas]

This is really cool, thank you!