Use a submodule for pydantic v1 compat (#9371)

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libs/experimental/langchain_experimental/pydantic_v1/__init__.py

@@ -0,0 +1,23 @@
+from importlib import metadata
+
+## Create namespaces for pydantic v1 and v2.
+# This code must stay at the top of the file before other modules may
+# attempt to import pydantic since it adds pydantic_v1 and pydantic_v2 to sys.modules.
+#
+# This hack is done for the following reasons:
+# * Langchain will attempt to remain compatible with both pydantic v1 and v2 since
+#   both dependencies and dependents may be stuck on either version of v1 or v2.
+# * Creating namespaces for pydantic v1 and v2 should allow us to write code that
+#   unambiguously uses either v1 or v2 API.
+# * This change is easier to roll out and roll back.
+
+try:
+    from pydantic.v1 import *  # noqa: F403
+except ImportError:
+    from pydantic import *  # noqa: F403
+
+
+try:
+    _PYDANTIC_MAJOR_VERSION: int = int(metadata.version("pydantic").split(".")[0])
+except metadata.PackageNotFoundError:
+    _PYDANTIC_MAJOR_VERSION = 0

libs/experimental/langchain_experimental/pydantic_v1/dataclasses.py

@@ -0,0 +1,4 @@
+try:
+    from pydantic.v1.dataclasses import *  # noqa: F403
+except ImportError:
+    from pydantic.dataclasses import *  # noqa: F403

libs/experimental/langchain_experimental/pydantic_v1/main.py

@@ -0,0 +1,4 @@
+try:
+    from pydantic.v1.main import *  # noqa: F403
+except ImportError:
+    from pydantic.main import *  # noqa: F403