feat: enable rules_python Starlark implemention by default for Bazel 7+ (bazel-contrib#1699)

This makes the base rules use the Starlark code in rules_python
("pystar") instead of what is built into Bazel 7.

The pystar implementation can be disabled by setting
`RULES_PYTHON_ENABLE_PYSTAR=0` in your environment or using
`--action_env`.

Earlier versions of Bazel continue to use the rules built into Bazel
regardless (the new implementation requires special APIs that only Bazel
7 has).

Work towards bazel-contrib#1069

Author: rickeylev

CHANGELOG.md

@@ -27,6 +27,12 @@ A brief description of the categories of changes:
 
 ### Changed
 
+* For Bazel 7, the core rules and providers are now implemented in rules_python
+  directly and the rules bundled with Bazel are not used. Bazel 6 and earlier
+  continue to use the Bazel builtin symbols. Of particular note, this means,
+  under Bazel 7, the builtin global symbol `PyInfo` is **not** the same as what
+  is loaded from rules_python. The same is true of `PyRuntimeInfo`.
+
 * (toolchains) Windows hosts always ignore pyc files in the downloaded runtimes.
   This fixes issues due to pyc files being created at runtime and affecting the
   definition of what files were considered part of the runtime.

python/private/internal_config_repo.bzl

@@ -19,7 +19,7 @@ settings for rules to later use.
 """
 
 _ENABLE_PYSTAR_ENVVAR_NAME = "RULES_PYTHON_ENABLE_PYSTAR"
-_ENABLE_PYSTAR_DEFAULT = "0"
+_ENABLE_PYSTAR_DEFAULT = "1"
 
 _CONFIG_TEMPLATE = """\
 config = struct(