import_failed false positive with relative imports

[mflova]

Given the following mock project:

root/
└── lib/
    ├── a.py
    └── b.py

# a.py
# Empty

# b.py
import a

For this example I can perfectly run b.py from both root/ or from lib/. However, when I launch pyanalyze from root/, I get import_failed code. Apparently, because it cannot import a from b.py

[JelleZijlstra]

This is probably fixable by setting the PYTHONPATH environment variable appropriately. Possibly pyanalyze itself could set up its import path better too.