feat: Show me warning if artificialtion is empty or I did not use the artificialtion.

## Feature Request

### Motivation

* Background:
Since I didn't know much about volo, I did a little exploring.
I added some entries, but I didn't notice that I added thrift without service.

### Proposal
1. So I want to do a [cargo check or cargo build or something] to remind me that the file I generated for the entry is empty.
In other word, just give me a warning.
2. Even, if I don't include the corresponding filename, give me a warning.

* more info about option 2 in lib.rs
```rs
mod gen {
volo::include_service!("volo_gen.rs");
volo::include_service!("volo_gen2.rs"); // if I forget add this one
}

pub use gen::volo_gen::*;
pub use gen::volo_gen2::*;
```