Enabling `keyword_idents` lints doesn't warn about violations in other files

Consider this tiny two-file crate:

// main.rs
#![deny(keyword_idents)]

mod submod;

fn main() {}

// submod.rs
fn async() {}
fn await() {}
fn try() {}
fn dyn() {}
fn gen() {}

Building this crate should fail with several violations of the keyword_idents_2018 and keyword_idents_2024 lints. But instead, building unexpectedly succeeds!

(This particular example requires Rust 2015, but the same issue can be observed in 2018/2021 regarding the gen keyword only.)

Reproduce the issue with the two-file Rust crate shown in main.rs and submod.rs, using Rust 2015 and the keyword_idents lints. Start by tracing how lint checking handles declarations in the submodule, then run the crate build to verify that violations in submod.rs are reported as expected.