Mention common resolutions for build errors

[hauntsaninja]

Might help with breaking changes that result from #9636

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

mitmproxy (https://github.com/mitmproxy/mitmproxy)
- examples/addons/anatomy.py: note: Are you missing an __init__.py? Alternatively, consider using --exclude to avoid checking one of them.
+ examples/addons/anatomy.py: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#mapping-file-paths-to-modules for more info
+ examples/addons/anatomy.py: note: Common resolutions include: a) using `--exclude` to avoid checking one of them, b) adding `__init__.py` somewhere, c) using `--explicit-package-bases` or adjusting MYPYPATH

[hauntsaninja]

Note that a common resolution currently is also to specify --namespace-packages, but I'm omitting that because of #9636