Establish pattern for checks

[jameslamb]

As of this writing, this project just downloads files and reports on their contents.

It should start raising warnings for things that package authors may want to enforce in continuous integration.

Rough initial set of ideas:

• found executable files
• total size > {some_threshold}
  • moved to new check: compressed size too large #53
• surprising / disallowed file extensions
  • [new check] unexpected files found #55
• included test files
  • I think this is redundant with [new check] unexpected files found #55
• wheel contains compiled objects/libraries with debug symbols
  • moved to [new check] wheel contains compiled objects/libraries with debug symbols #54

And I think it would be cool for those warnings to have unique identifiers that map to documentation and configuration, similar to what shellcheck, hadolint, and pylint do. e.g. https://github.com/koalaman/shellcheck/wiki/SC2086.