Make follow-up improvements to `validate_record`

[pradyunsg]

There's also a few more changes to make around this (as a follow up to #105):

• rename the argument to validate_contents, and make it kw-only.
• should validate that everything has a hash/size even if validate_contents is False.
• should validate that RECORD signatures are not mentioned in the RECORD.
• should validate that the RECORD file is mentioned without hash or size.
• documentation needs to present an example and add cross-references to/from WheelSource.validation_error.

[pradyunsg]

FWIW, I'm also not a big fan of the inconsistency within test_sources.py around function tests vs method tests and would prefer to unify things under the TestWheelFile class.

[pradyunsg]

Thanks @BlueGlassBlock! You're awesome! ❤️