Add a better error message when mypy complains about duplicate package definitions (e.g. if "build/" dir exists)

[obi1kenobi]

If a user releases their package, they may have build/ and/or dist/ directories left over in their project directory. The existence of the duplicate source code files will make mypy return an obscure error message.

We should detect the fact that mypy raised the error because of this very common situation, and produce a better and more actionable user-facing error message.