fix(templates): resolve core dependencies locally and batch pip installs

[ohmayr]

fix(templates): resolve core dependencies locally and batch pip installs

Previously, the core_deps_from_source and prerelease_deps nox sessions installed core sibling packages sequentially. In a monorepo environment, this caused two major issues:

1. Correctness: It frequently fetched remote upstream code (or PyPI releases) instead of testing the actual local code modified in the active PR.
2. Performance: Sequential pip install commands triggered strict dependency resolution repeatedly, leading to severe CI timeouts.

This updates the noxfile template to:

• Dynamically resolve first-party dependencies from the local packages/ directory to guarantee the presubmit tests the active PR's code.
• Batch the pip install commands using --no-deps and --ignore-installed to bypass the resolver overhead and eliminate the sequential network bottleneck.
• Dynamically sort packages in prerelease_deps into local vs. PyPI deployments using a regex parser, ensuring safe handling of complex version bounds (e.g., grpcio>=1.75.1) without hardcoding multiple lists.

[gemini-code-assist]

Code Review

This pull request updates the noxfile.py template and its corresponding golden files to install core dependencies from local paths instead of remote git repositories. Feedback indicates that a comment needs updating to reflect the new variable name core_packages. Additionally, the logic for locating the packages directory is currently too simplistic and will fail for nested golden files; a more robust traversal method was suggested to ensure the correct path is resolved.