Implement foundational validation logic

[pradyunsg]

This would be something like:

installer.validators.validate_all_files_match_record(source: WheelSource) -> None
installer.validators.validate_record(source: WheelSource) -> None

These raise installer.validators.ValidationError if the source is not a valid wheel.

[pradyunsg]

Closing this since #147 did the basics and #156 covers the rest needed around this.