break up req.py into a package

this breaks `req.py` into a package like so:

```
req/
   __init__.py          # has an __all__ that preserves major imports
   req_set.py           # main item:   RequirementSet
   req_file.py           # main item:    parse_requirements
   req_install.py       # main item:   InstallRequirement
   req_uninstall.py   # main item:   UninstallPathSet
```