Integrate the `deps` attribute of targets into MyPy's module searching behaviour

[thundergolfer]

I think if a dependency isn't properly included in a target's deps list, then the MyPy aspect shouldn't circumvent Bazel and try and find that dependency in order to include it in type-checking; it should instead not look at the dependency and consider it missing.

[thundergolfer]

Pretty sure this is now implemented and working. I have a folder for testing it: examples/testing_deps.