chore(package): migrate to src/ layout (#3920)

Migrates package structure to src/ layout following Python packaging
best practices.

All code moved from `llama_stack/` to `src/llama_stack/`. Public API
unchanged - imports remain `import llama_stack.*`.

Updated build configs, pre-commit hooks, scripts, and GitHub workflows
accordingly. All hooks pass, package builds cleanly.

**Developer note**: Reinstall after pulling: `pip install -e .`

src/llama_stack/providers/remote/inference/groq/groq.py

@@ -0,0 +1,18 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+
+from llama_stack.providers.remote.inference.groq.config import GroqConfig
+from llama_stack.providers.utils.inference.openai_mixin import OpenAIMixin
+
+
+class GroqInferenceAdapter(OpenAIMixin):
+    config: GroqConfig
+
+    provider_data_api_key_field: str = "groq_api_key"
+
+    def get_base_url(self) -> str:
+        return f"{self.config.url}/openai/v1"