`proto3` doesn't pack repeated fields by default

The code here https://github.com/tokio-rs/prost/blob/63c00248abef84337c1010b6a10f3258b2db8b54/prost-build/src/code_generator.rs#L435 attempts to set the default behaviour for `proto3` to be packed, however because `options.packed()` returns `false` unless `[packed = true]` is set on the field, this fails.