pystar: support builtin providers for compatibility (#1573)

This makes the rules_python Starlark implementation accept and return
the builtin providers. This allows depending on, and being depended on
by, the builtin rules, which enables the two rule sets to interoperate
better.

Work towards #1069
diff --git a/python/private/common/py_library.bzl b/python/private/common/py_library.bzl
index 8d09c51..28ee7bf 100644
--- a/python/private/common/py_library.bzl
+++ b/python/private/common/py_library.bzl
@@ -61,7 +61,7 @@
     runfiles = collect_runfiles(ctx = ctx, files = output_sources)
 
     cc_info = semantics.get_cc_info_for_library(ctx)
-    py_info, deps_transitive_sources = create_py_info(
+    py_info, deps_transitive_sources, builtins_py_info = create_py_info(
         ctx,
         direct_sources = depset(direct_sources),
         imports = collect_imports(ctx, semantics),
@@ -78,6 +78,7 @@
     return [
         DefaultInfo(files = output_sources, runfiles = runfiles),
         py_info,
+        builtins_py_info,
         create_instrumented_files_info(ctx),
         PyCcLinkParamsProvider(cc_info = cc_info),
         create_output_group_info(py_info.transitive_sources, extra_groups = {}),