Normalize wheel_download_dir passed to RequirementSet

This handles `wheel_dir = ~/.cache/pip/wheelhouse` correctly when
invoked via `pip wheel -r requirements.txt`.
Use `normalize_path` for `download_cache` passed to RequirementSet

This appears to be in line with other places where paths are normalized.
Merge pull request #1788 from blueyed/requirementset-normalize-paths

RequirementSet: normalize paths (esp. wheel_download_dir)