Correct handling of fat3 macOS wheels and eggs.

[freakboy3742]

Fixes #17566.

Warehouse currently accepts wheels of type fat32; this tag doesn't exist, and appears to be a typo of fat3. There are no examples of fat32 wheels in the Warehouse database (as Python tooling has never produced them, and pip would never request them).

It also labels eggs as "Source" (see https://pypi.org/project/pillow/2.5.0/#files for an example).

This PR:

• Corrects the fat3 wheel tag
• Adds architecture descriptors for the macOS combined architectures other than universal2
• Ensures eggs are described as such
• Adds tests for all of the above.

These changes are almost entirely anachronistic... but they are part of the spec, and the discrepancy (discovered during writing pypa/packaging.python.org#1804) was just itching me :-)

[di]

Thanks!