Track local variables and scopes

[benjaminjkraft]

There are several cases (e.g. #2) where we confuse local variables with imported symbols of the same name, or where we make sloppy assumptions about where a late-import's alias is available. The correct thing to do is keep track of scopes and what imports and variable assignments happen in each. This is potentially complex but there are a number of edge cases we can't handle perfectly without it.