Import common proto in each crate?

### Version

```
tonic v0.9.2
tonic v0.9.2 (*)
tonic-build v0.9.2
tonic v0.9.2 (*)
tonic-web v0.9.2
tonic v0.9.2 (*)
```

### Platform

Win 10 64

### Description

Hi people! Thanks for your amazing work!

I have this dir structure:

```
- Cargo.toml (workspace)
- src
- common
- Cargo.toml
- src
- common.proto

- crates
- players
- Cargo.toml
- build.rs
- src
- proto
- players.proto (I would like to import src/common/src/common.proto here)

- teams
- Cargo.toml
- build.rs
- src
- proto
- teams.proto (I would like to import src/common/src/common.proto here)

- others...
```

as you can see I have many crates under `crates` dir and I have a `common.proto` which I would like to import in my crates.

I'm using a `build.rs` for each crate because I cannot (right?) use one for the entire Cargo workspace (I tried: Cargo doesn't start build.rs in workspace root dir).

So I'm trying to import common.proto in file like this:

```proto
syntax = "proto3";

package players;

import "google/protobuf/timestamp.proto";

import "src/common/src/common.proto";
```

and in my VSCode this is working.

But when I start `build.rs` throws with:

```
error: failed to run custom build command for `players v0.1.0`

Caused by:
process didn't exit successfully: `C:\prj\target\debug\build\orders-b13065cbe3fac50f\build-script-build` (exit code: 1)
--- stdout
cargo:rerun-if-changed=src/proto/players.proto
cargo:rerun-if-changed=src/proto

--- stderr
Error: Custom { kind: Other, error: "protoc failed: src/common/src/common.proto: File not found.\r\nplayers.proto:6:1: Import \"src/common/src/common.proto\" was not found or had errors.\r\nplayers.proto:29:3: \"common.Pagination\" is not defined.\r\nplayers.proto:49:9: \"common.Page\" is not defined.\r\n" }
[Finished running. Exit status: 0]
```

My `build.rs` is:

```rust
fn main() -> Result<(), Box> {
tonic_build::configure()
.out_dir("src/proto")
.compile(
&["src/proto/players.proto"],
&["src/proto"],
)?;

Ok(())
}
```

How can I fix this?