Feature

When mypy is run against a package that's installed but isn't PEP-561 compliant, it will report that it can't find the package.

It would be better to state that the package is there, but it's not PEP-561 compliant.

Pitch

The "package not found" message is extremely confusing the first (and even the fifth) time it's encountered. Developers can waste a lot of time trying to figure out why mypy can't find the package because "it's right there! I just installed it! I can import it!"