Check that name is defined [name-defined]

Mypy expects that all references to names have a correspondingdefinition in an active scope, such as an assignment, functiondefinition or an import. This can catch missing definitions, missingimports, and typos.

This example accidentally calls sort() instead of sorted():

x = sort([3, 2, 4])  # Error: Name 'sort' is not defined  [name-defined]