Better error messages for invalid Python package directory names #2775

It looks like mypy can't handle absolute paths to where the parent directory has a dot in the name. Specifically I accidentally had an __init__.py in the root of my project, and cloned my project into a directory with a dot in its name. When I ran mypy it failed with mypy: can't find module '<module-name>'. I realize that what I was doing was wrong, but it took me cloning mypy and stepping through it with a debugger to understand what was wrong and why it was failing. It would be awesome to have a clearer error message in these cases (eg maybe check that the directories found in the crawl_up method are valid package names and fail if they're not:

To repro, create a directory with a dot in it (eg mkdir tmp.dir), place an empty __init__.py and an empty main.py in it, then run mypy $(pwd)/main.py from within that directory.

Note that this is related to but slightly different from #2773.