fix(py_image_layer): Fix interpreter regex matching pip packages

[arrdem]

The interpreter layer regex in py_image_layer was too broad — it matched any runfiles path segment containing both "python" and an architecture string. Pip packages with arch tags in their canonical repo name (e.g. rules_python++pip+pypi_313_argon2_cffi_bindings_cp36_abi3_manylinux_2_17_x86_64_...) were incorrectly placed in the interpreter layer.

New regex matches the interpreter repo naming convention: python[_]<major>_<minor>[_<patch>]_<arch>[-_]<vendor>[-_]<os>, keying on the platform triplet vendor field (unknown/apple/pc). Handles:

• rules_python style with hyphens (python_3_9_x86_64-unknown-linux-gnu)
• aspect_rules_py provisioned style with underscores (python_3_11_x86_64_unknown_linux_gnu)
• Both bzlmod (+) and WORKSPACE (~) module separators

Fixes #787

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

Release note: Fix py_image_layer incorrectly placing pip packages with architecture strings in their name into the interpreter layer instead of the packages layer.

Test plan

• New test: //py/tests/py_image_layer_regex:test_interpreter_regex — 19 assertions covering interpreter repos (both naming conventions), pip packages, and main repo paths
• Covered by existing e2e OCI image layer tests

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✅ arrdem
❌ aspect-marvin
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