sometimes make private names available

*Within* a single package, sometimes names beginning with underscores are things that need to be imported internally; implementation modules might be private. So if I'm working in `src/foo/_bar.py` and `_baz` is defined in `src/foo/_qux.py`, I want to see that import available. However, not all private symbols should be indexed; if I'm in `src/xyz`, I should not see any private imports from `foo/`.