chore: remove non-toolchain runtime resolution logic. (bazel-contrib#3280)

Finding the Python runtime via toolchain resolution has been enabled by
default for
many years. Toolchains are the preferred mechanism for specifying Python
runtime info.
Usage of the non-toolchain ways should be approximately zero, and they'd
be considered
unsupported anyways. Hence this is not considered a breaking change.

This PR removes support for

- "If toolchains disabled" logic
(`--incompatible_use_python_toolchains`)
- `configuration_field` on `python_top` (`--python_top` flag)
- `_py_interpreter` attribute (plumbing for python_top flag)

Work towards bazel-contrib#3252.

---------

Co-authored-by: Richard Levasseur <[email protected]>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: 3 people

python/private/py_executable.bzl

@@ -59,7 +59,7 @@ load(":py_cc_link_params_info.bzl", "PyCcLinkParamsInfo")
 load(":py_executable_info.bzl", "PyExecutableInfo")
 load(":py_info.bzl", "PyInfo", "VenvSymlinkKind")
 load(":py_internal.bzl", "py_internal")
-load(":py_runtime_info.bzl", "DEFAULT_STUB_SHEBANG", "PyRuntimeInfo")
+load(":py_runtime_info.bzl", "DEFAULT_STUB_SHEBANG")
 load(":reexports.bzl", "BuiltinPyInfo", "BuiltinPyRuntimeInfo")
 load(":rule_builders.bzl", "ruleb")
 load(":toolchain_types.bzl", "EXEC_TOOLS_TOOLCHAIN_TYPE", "TARGET_TOOLCHAIN_TYPE", TOOLCHAIN_TYPE = "TARGET_TOOLCHAIN_TYPE")
@@ -203,15 +203,6 @@ accepting arbitrary Python versions.
             # empty target for other platforms.
             default = "//tools/launcher:launcher",
         ),
-        "_py_interpreter": lambda: attrb.Label(
-            # The configuration_field args are validated when called;
-            # we use the precense of py_internal to indicate this Bazel
-            # build has that fragment and name.
-            default = configuration_field(
-                fragment = "bazel_py",
-                name = "python_top",
-            ) if py_internal else None,
-        ),
         # TODO: This appears to be vestigial. It's only added because
         # GraphlessQueryTest.testLabelsOperator relies on it to test for
         # query behavior of implicit dependencies.
@@ -1202,41 +1193,28 @@ def _maybe_get_runtime_from_ctx(ctx):
     Returns:
         2-tuple of toolchain_runtime, effective_runtime
     """
-    if ctx.fragments.py.use_toolchains:
-        toolchain = ctx.toolchains[TOOLCHAIN_TYPE]
-
-        if not hasattr(toolchain, "py3_runtime"):
-            fail("Python toolchain field 'py3_runtime' is missing")
-        if not toolchain.py3_runtime:
-            fail("Python toolchain missing py3_runtime")
-        py3_runtime = toolchain.py3_runtime
-
-        # Hack around the fact that the autodetecting Python toolchain, which is
-        # automatically registered, does not yet support Windows. In this case,
-        # we want to return null so that _get_interpreter_path falls back on
-        # --python_path. See tools/python/toolchain.bzl.
-        # TODO(#7844): Remove this hack when the autodetecting toolchain has a
-        # Windows implementation.
-        if py3_runtime.interpreter_path == "/_magic_pyruntime_sentinel_do_not_use":
-            return None, None
-
-        if py3_runtime.python_version != "PY3":
-            fail("Python toolchain py3_runtime must be python_version=PY3, got {}".format(
-                py3_runtime.python_version,
-            ))
-        toolchain_runtime = toolchain.py3_runtime
-        effective_runtime = toolchain_runtime
-    else:
-        toolchain_runtime = None
-        attr_target = ctx.attr._py_interpreter
-
-        # In Bazel, --python_top is null by default.
-        if attr_target and PyRuntimeInfo in attr_target:
-            effective_runtime = attr_target[PyRuntimeInfo]
-        else:
-            return None, None
-
-    return toolchain_runtime, effective_runtime
+    toolchain = ctx.toolchains[TOOLCHAIN_TYPE]
+
+    if not hasattr(toolchain, "py3_runtime"):
+        fail("Python toolchain field 'py3_runtime' is missing")
+    if not toolchain.py3_runtime:
+        fail("Python toolchain missing py3_runtime")
+    py3_runtime = toolchain.py3_runtime
+
+    # Hack around the fact that the autodetecting Python toolchain, which is
+    # automatically registered, does not yet support Windows. In this case,
+    # we want to return null so that _get_interpreter_path falls back on
+    # --python_path. See tools/python/toolchain.bzl.
+    # TODO(#7844): Remove this hack when the autodetecting toolchain has a
+    # Windows implementation.
+    if py3_runtime.interpreter_path == "/_magic_pyruntime_sentinel_do_not_use":
+        return None, None
+
+    if py3_runtime.python_version != "PY3":
+        fail("Python toolchain py3_runtime must be python_version=PY3, got {}".format(
+            py3_runtime.python_version,
+        ))
+    return py3_runtime, py3_runtime
 
 def _get_base_runfiles_for_binary(
         ctx,