When I'm using tonic_build with slight complex structure, the code generator display weird errors

## Bug Report

When I'm using tonic_build with slight complex structure, the code generator display weird errors

### Version

└── tonic v0.8.0
└── tonic-build v0.8.0

### Platform

21.5.0 Darwin Kernel Version 21.5.0

### Description

In my proto folder I have different type of subfolder each one containing multiple proto files with the same namespace. Now I want to import some of them into other and compile everything with tonic_build but I get weird errors. I created a PoC for the error, and the link to the GitHub repository is the following one: https://github.com/fbrv/failing-example-tonic