Implement unused import warning, and lowering of source code info extensions

[mcy]

This PR adds two related features:

1. An unused imports warning, which requires additional book-keeping to track which direct import caused a transitive import to get imported.
2. Codegen for the Buf source code info extension, which includes unused imports and whether a syntax production exists in the file.

In the process of implementing this, I found a bug in how imports are built, which this PR fixes. It showed up because import causes weren't being listed correctly, leading to incorrect diagnosis of unused imports. I also fixed a bug in yaml.go where extensions we're being printed in the output.

[mcy]

There is one broken test due to a bug in how editions are lowered. See #567