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/core/external.py

@@ -0,0 +1,54 @@
+# 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.
+
+
+import yaml
+
+from llama_stack.apis.datatypes import Api, ExternalApiSpec
+from llama_stack.core.datatypes import BuildConfig, StackRunConfig
+from llama_stack.log import get_logger
+
+logger = get_logger(name=__name__, category="core")
+
+
+def load_external_apis(config: StackRunConfig | BuildConfig | None) -> dict[Api, ExternalApiSpec]:
+    """Load external API specifications from the configured directory.
+
+    Args:
+        config: StackRunConfig or BuildConfig containing the external APIs directory path
+
+    Returns:
+        A dictionary mapping API names to their specifications
+    """
+    if not config or not config.external_apis_dir:
+        return {}
+
+    external_apis_dir = config.external_apis_dir.expanduser().resolve()
+    if not external_apis_dir.is_dir():
+        logger.error(f"External APIs directory is not a directory: {external_apis_dir}")
+        return {}
+
+    logger.info(f"Loading external APIs from {external_apis_dir}")
+    external_apis: dict[Api, ExternalApiSpec] = {}
+
+    # Look for YAML files in the external APIs directory
+    for yaml_path in external_apis_dir.glob("*.yaml"):
+        try:
+            with open(yaml_path) as f:
+                spec_data = yaml.safe_load(f)
+
+            spec = ExternalApiSpec(**spec_data)
+            api = Api.add(spec.name)
+            logger.info(f"Loaded external API spec for {spec.name} from {yaml_path}")
+            external_apis[api] = spec
+        except yaml.YAMLError as yaml_err:
+            logger.error(f"Failed to parse YAML file {yaml_path}: {yaml_err}")
+            raise
+        except Exception:
+            logger.exception(f"Failed to load external API spec from {yaml_path}")
+            raise
+
+    return external_apis