Support local (relative) import paths

We haven't directly supported these before now. They may or may not work, but they're not tested.

We could resolve relative import paths by passing the compiled library's import path to the builder. We might already pass this to the compiler as `-p`.

Start by tracing how the builder passes the compiled library's import path to the compiler, including the existing `-p` argument. Determine where relative import paths are resolved, then add coverage for local imports and verify that those imports build successfully.

Written by the indexing model from the issue text.