Move __pip-runner__ script into a module

[pradyunsg]

Follow up to #11257

[pradyunsg]

Moving this out into a module helped me identify that we weren't correctly replacing the pip module in the runner script, which meant that the script wasn't using the correct pip copy! I'm not quite sure why our tests didn't catch that, and I'm looking at the test suite to figure out how to write a test for this.

I've changed the implementation, in this PR, to ensure that we replace that module correctly.

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One nice property of moving this into a module, is that I can run it for testing things manually (and automatically)!

Before:

❯ python -m pip --version
/Users/pradyunsg/Developer/github/pip/.venv/bin/python: No module named pip
❯ python src/pip/__pip-runner__.py install .   
Traceback (most recent call last):
  File "/Users/pradyunsg/Developer/github/pip/src/pip/__pip-runner__.py", line 37, in <module>
    runpy.run_module("pip", run_name="__main__")
  File "/Users/pradyunsg/.asdf/installs/python/3.10.1/lib/python3.10/runpy.py", line 205, in run_module
    mod_name, mod_spec, code = _get_module_details(mod_name)
  File "/Users/pradyunsg/.asdf/installs/python/3.10.1/lib/python3.10/runpy.py", line 140, in _get_module_details
    raise error("No module named %s" % mod_name)
ImportError: No module named pip

After:

❯ python -m pip --version
/Users/pradyunsg/Developer/github/pip/.venv/bin/python: No module named pip
❯ python src/pip/__pip-runner__.py install .
Processing /Users/pradyunsg/Developer/github/pip
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Preparing metadata (pyproject.toml) ... done
Building wheels for collected packages: pip
  Building wheel for pip (pyproject.toml) ... done
  Created wheel for pip: filename=pip-22.2.dev0-py3-none-any.whl size=2145280 sha256=9c06ef8f4a063b041160ed4e843e570c4564549754441d2502ff4c9cb348092f
  Stored in directory: /private/var/folders/y1/j465wvf92vs938kmgqh63bj80000gn/T/pip-ephem-wheel-cache-4ofpbybe/wheels/54/9f/74/29320177d01b5bd98b902e57e24b770257a633ca58c02b60f1
Successfully built pip
Installing collected packages: pip
Successfully installed pip-22.2.dev0
❯ python -m pip --version
pip 22.2.dev0 from /Users/pradyunsg/Developer/github/pip/.venv/lib/python3.10/site-packages/pip (python 3.10)