Summary

Follow-up to #18176. This fixes the remaining absolute/relative path issues when uv add edits pyproject.toml.

uv add now preserves the user's path form:

• Relative paths remain relative.
• Absolute paths and literal file:// URLs remain absolute.
• URLs containing expanded variables remain relative, preserving the portability behavior from Treat paths originating as PEP 508 URLs which contain expanded variables as relative #18680 for both named and unnamed requirements.

This applies to local directories and local archives.

• Closes uv add results in relative path for a local package #17307
• Closes Preserve absolute paths in uv add for local dependencies #17316

Test Plan

• Integration coverage for relative paths, absolute paths, and literal file:// URLs.
• Integration coverage for directory and archive sources.
• Regression coverage for expanded-variable URLs.
• cargo nextest run -E 'test(add_relative_and_absolute_paths) or test(add_relative_and_absolute_archives) or test(lock_pep508_urls_with_vars)'
• cargo nextest run -p uv-pep508 -p uv-pypi-types
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo xwin clippy --workspace --all-targets --all-features --locked -- -D warnings