refactor(exec_tools): hoist py_exec_tools_toolchain from rules_python, drop min version to 1.0.0

[gregmagolan]

Summary

#903 bumped the minimum rules_python version from 1.0.0 to 1.9.0 to gain access to PyExecToolsInfo.exec_runtime (added in rules_python 1.9.0) and loaded py_exec_tools_toolchain from rules_python's private API. This PR reverses the version bump by hoisting the necessary code into rules_py and removes all imports from rules_python/private.

Changes

New py/private/exec_tools/defs.bzl — hoisted from rules_python:

• PyExecToolsInfo provider with only an exec_runtime field. We own the provider so we can include exec_runtime (the field added in 1.9.0) regardless of which rules_python version the user has.
• py_exec_tools_toolchain rule — instantiated in each PBS interpreter repo; derives exec_runtime from exec_interpreter[ToolchainInfo].py3_runtime (the pre-1.9.0 equivalent).
• current_interpreter_executable rule — resolves PY_TOOLCHAIN to create an exec-configured interpreter symlink; defaults for the above rule.

py/private/toolchain/BUILD.bazel — added exec_tools_toolchain_type toolchain type. We declare our own type rather than reusing @rules_python//python:exec_tools_toolchain_type, so our PyExecToolsInfo never needs to satisfy rules_python's internal interface (e.g. the precompiler field that maybe_precompile checks for).

py/private/toolchain/types.bzl — EXEC_TOOLS_TOOLCHAIN now points to @aspect_rules_py//py/private/toolchain:exec_tools_toolchain_type.

py/private/interpreter/repository.bzl — generated interpreter BUILD files now load py_exec_tools_toolchain from @aspect_rules_py//py/private/exec_tools:defs.bzl and register under our own toolchain type instead of @rules_python//python:exec_tools_toolchain_type.

uv/private/whl_install/rule.bzl — EXEC_TOOLS_TOOLCHAIN now loaded from our own types.bzl.

MODULE.bazel / e2e/MODULE.bazel — rules_python and rules_python_gazelle_plugin dropped back to 1.0.0.

[aspect-workflows]

Bazel 8 (Test)

All tests were cache hits

111 tests (100.0%) were fully cached saving 53s.

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Bazel 9 (Test)

All tests were cache hits

111 tests (100.0%) were fully cached saving 1m 20s.

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Bazel 8 (Test)

e2e

All tests were cache hits

51 tests (100.0%) were fully cached saving 55s.

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Bazel 9 (Test)

e2e

All tests were cache hits

51 tests (100.0%) were fully cached saving 43s.

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Bazel 8 (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 444ms.

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