More build and tests with Bzlmod enabled

- Build `//src:bazel`
- Test `//src/test/cpp/...`

PiperOrigin-RevId: 523426054
Change-Id: I41d9875d9acf5e44f667d672d6aebccbd8e5350e
diff --git a/extensions.bzl b/extensions.bzl
new file mode 100644
index 0000000..800ee5f8
--- /dev/null
+++ b/extensions.bzl
@@ -0,0 +1,24 @@
+# Copyright 2023 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Module extensions for loading dependencies we need to build Bazel.
+
+"""
+
+load("//:repositories.bzl", "embedded_jdk_repositories")
+
+def _bazel_internal_deps(_ctx):
+    embedded_jdk_repositories()
+
+bazel_internal_deps = module_extension(implementation = _bazel_internal_deps)