fix(py_venv): Implement collision handling

[arrdem]

This patch extends the new static venv machinery to handle conflicts in the logical venv path. Support is preliminary but complete, a multi-pass venv creation strategy would be better for producing comprehensive errors rather than forcing users to whack-a-mole.

Fixes #575

Changes are visible to end-users: yes

• 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: yes

Test plan

• New test cases added
• error mode conflicting venv is broken
• warn mode conflicting venv produces a warning
• ignore mode conflicting venv is silent
• "Conflicting" files with same content are handled silently

[aspect-workflows]

Test

27 test targets passed

Targets

//examples/multi_version:py_version_default_test [k8-fastbuild]                             1s
//examples/multi_version:py_version_test [k8-fastbuild-ST-494921797612]                     2s
//examples/pytest:pytest_test [k8-fastbuild]                                                2s
//examples/pytest:sharded/test [k8-fastbuild]                                               3s
//examples/virtual_deps:pytest_test [k8-fastbuild]                                          1s
//py/tests/cc-deps:test_smoke [k8-fastbuild]                                                495ms
//py/tests/external-deps/sibling-package:test_test [k8-fastbuild]                           41ms
//py/tests/external-deps:test_0_test [k8-fastbuild]                                         75ms
//py/tests/external-deps:test_1_test [k8-fastbuild]                                         81ms
//py/tests/external-deps:test_can_import_runfiles_helper [k8-fastbuild]                     549ms
//py/tests/internal-deps:assert [k8-fastbuild]                                              714ms
//py/tests/py-binary:runfiles_from_pip_test [k8-fastbuild]                                  713ms
//py/tests/py-binary:test__main_from_name [k8-fastbuild]                                    32ms
//py/tests/py-binary:test__run_py_test_help [k8-fastbuild]                                  31ms
//py/tests/py-external-venv:test [k8-fastbuild]                                             200ms
//py/tests/py-internal-venv:test [k8-fastbuild]                                             363ms
//py/tests/py-pex-binary:test__print_modules_pex [k8-fastbuild]                             24ms
//py/tests/py-test:test_env_vars [k8-fastbuild]                                             636ms
//py/tests/py_image_layer:my_app_layers_test_test [k8-fastbuild]                            36ms
//py/tests/py_image_layer:py_image_test [k8-fastbuild]                                      5s
//py/tests/py_venv_conflict:py_venv_conflict [k8-fastbuild]                                 54ms
//py/tests/py_venv_image_layer:my_app_amd64_layers_test_test [k8-fastbuild]                 40ms
//py/tests/py_venv_image_layer:my_app_arm64_layers_test_test [k8-fastbuild]                 52ms
//py/tests/py_venv_image_layer:py_amd64_image_command_test [k8-fastbuild]                   2s
//py/tests/py_venv_image_layer:py_amd64_image_content_test [k8-fastbuild]                   2s
//py/tests/py_venv_image_layer:py_arm64_image_content_test [k8-fastbuild]                   3s
//py/tests/repo_relative_imports/test:test [k8-fastbuild]                                   716ms

Total test execution time was 26s. 12 tests (30.8%) were fully cached saving 45s.

[github-actions]

e2e/use_release folder: LCOV of commit 222f9f5 during CI #1820

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