Allow pip to show files installed for wheels

- shortened looping logic in search_packages_info
- added logic to find installed files for .dist-info's
- Seemed to be off-by-one error for assert exceptions in test_show_with_files_not_found
- changed ..._with_files_not_found to use an editable install for being unlistable
- added test to assert a wheel's files are now listed :D

fixes #1484