fix(core): place repack dependencies in code root

[lucasjia-aws]

Summary

Repacked models currently place user dependencies under code/lib, while SageMaker inference toolkits add only code to the Python path. This change copies file and directory dependencies directly into code, matching source-directory packaging and making the dependencies importable at runtime.

The existing collision behavior is preserved: directory conflicts raise FileExistsError, while file dependencies retain shutil.copy2 overwrite semantics.

Testing

• Added coverage for directory dependencies, single-file dependencies, coexistence with source directories, directory collisions, and file overwrite collisions.
• Ran the affected helper suite: 10 passed.
• Black, Flake8, and git diff --check passed.
• Built the sagemaker-core wheel and source distribution successfully.

Related change

The companion V2 fix is published #6461 .

Fixes #3491

[github-actions]

🤖 AI code review

This PR changes _create_or_update_code_dir in sagemaker-core so repack dependencies are copied directly into the code root rather than code/lib, matching source-directory packaging and the behavior described in issue #3491 (Source: #3491). The change is minimal and correct: copytree targets code_dir/basename for directories and copy2 targets code_dir for files, preserving the existing collision semantics. New tests cover directory deps, file deps, source-dir coexistence, directory collision, and file overwrite. I found no correctness or test gaps; the PR looks clean.

_{Reviewed commit 7abcaad. Automated review; verify before acting.}