Stable sorting of requirements.txt in universal mode (#5334)
The `RequirementsTxtComparator` was written assuming there is one
distribution per package name. This changed with the universal
resolution, which allows multiple versions or urls for the same package
name. The sorting we emitted for these new entries was incidental.
With this change, we properly sort these entries by name, version and
then url in universal mode.
This is an output format change for `--universal` users.
