Refactor validate_record

[BlueGlassBlock]

Address #156

• Moved tests to TestWheelFile
• 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]

Thanks for filing this @BlueGlassBlock! I'll merge this since this gets us most of the way there and we can tackle the docs in a follow up! :)