fix: namespace package calculation on windows

[rickeylev]

Currently, when the repo computes the namespace package files on
Windows, it incorrectly computes the filename because of C: handling
and case-sensitivity, resulting in a repo-phase error.

To fix, modify the relative path computation functions to detect if the
platform is Windows and make comparisions case-insensitive.

[gemini-code-assist]

Code Review

This pull request refactors path handling in repo_utils.bzl by introducing centralized normalization and relative path calculation functions, which are then utilized in pypi_repo_utils.bzl. However, the implementation of _is_relative_to contains reversed logic that incorrectly identifies paths as being outside the repository root. Additionally, _relative_to fails to preserve original casing on Windows and introduces unexpected trailing slashes, which may break downstream tools and imports.