tower-grpc-build does not support packages nested under a root package

[kevindrosendahl]

Hi, I've been able to reproduce an issue I've seen with generating services based on a package that contains a service under its root, and messages consumed by the service under sub-packages in the same package.

I've added a test case to reproduce the issue here: https://github.com/kevindrosendahl/tower-grpc/tree/2816fe19250c80612bca5af8fb5bd72497e77de1/tests/nested-package

It's the same test case as collide except that all of the other messages all exist in the hello
top level package (e.g. proto/common.proto is in package hello.common instead of common, proto/nested.proto is in package hello.common.nested).

Here is a gist of the output when running cargo build -p nested-package as well as the generated hello.rs: https://gist.github.com/kevindrosendahl/ebee338f5e8d84fefca8f8eb0fd8a428

From a read over the output, it appears that there are two issues at play:

1. Import name conflict (https://gist.github.com/kevindrosendahl/ebee338f5e8d84fefca8f8eb0fd8a428#file-hello-rs-L9 and https://gist.github.com/kevindrosendahl/ebee338f5e8d84fefca8f8eb0fd8a428#file-hello-rs-L11 both imported as HelloRequest)
2. The import for the methods trait appears to be looking for the common module in the server module rather than its parent, as the server module is properly doing (https://gist.github.com/kevindrosendahl/ebee338f5e8d84fefca8f8eb0fd8a428#file-hello-rs-L265).

I'm not sure how trivial this is to fix. If it seems like it may be a bit involved and you're unable to prioritize it, I can potentially try looking into contributing a solution in the coming weeks, but would probably need some guidance.

Thanks!