fix: write __init__ shim for pip runfiles helper

[thesayyn]

Fixes #377

Even though messing with init.py is a little risky, in this case its safer since we only generate it while symlinking files from external repositories into venv, and since /~https://github.com/bazelbuild/rules_python/blob/18f76f9cc4c3c6c4470f7a881e6ea4e8b80d7bab/python/runfiles/__init__.py#L15 is simple line, i can't think of easier fix than this.

Changes are visible to end-users: no

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: no

Test plan

• New test cases added

[github-actions]

e2e/use_release folder: LCOV of commit a89e960 during CI #1657

Summary coverage rate:
  lines......: 100.0% (2 of 2 lines)
  functions..: 100.0% (1 of 1 function)
  branches...: no data found

Files changed coverage rate: n/a

[aspect-workflows]

Test

10 test targets passed

Targets

//examples/multi_version:py_version_default_test [k8-fastbuild]                         1s
//examples/multi_version:py_version_test [k8-fastbuild-ST-494921797612]                 1s
//examples/pytest:nested/pytest [k8-fastbuild]                                          2s
//examples/pytest:pytest_test [k8-fastbuild]                                            2s
//examples/virtual_deps:pytest_test [k8-fastbuild]                                      1s
//py/tests/external-deps:test_can_import_runfiles_helper [k8-fastbuild]                 741ms
//py/tests/internal-deps:assert [k8-fastbuild]                                          460ms
//py/tests/py-binary:runfiles_from_pip_test [k8-fastbuild]                              968ms
//py/tests/py-test:test_env_vars [k8-fastbuild]                                         416ms
//py/tests/repo_relative_imports/test:test [k8-fastbuild]                               490ms

Total test execution time was 10s. 19 tests (65.5%) were fully cached saving 36s.