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

[huang12zheng]

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

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::*;

[PureWhiteWu]

For proposal 1: You can just check if the given entry is empty when calling volo-build, I think that's not hard. Would you like to send a pr for that?

For proposal 2: I'm not quite sure how to get this, to achieve the most accurate way we need to parse the whole crate, and we also know little about how the user want to use volo. For example, the user may not want to include the files in mod gen, rather he/she want to include the files in another mod called thrift_gen, and in that situation what should we do?