feat: add ignore_missing_submod option

[tanzaku]

Resolve #5609

Made the changes suggested by ytmimi in issue #5609.

This is my first contribution, so I may have made some mistakes. I would be happy to fix them if you point them out.

[ytmimi]

Thanks for your first contribution to rustfmt 🎉. Always great to see new contributors.

I won't have a chance to review this today, but I'll be sure to give feedback once I take a look!

[ytmimi]

Can we also add the following tests to tests/rustfmt/main.rs

#[test]
fn ignore_missing_sub_mod_false() {
    // Ensure that missing submodules cause module not found errors when trying to
    // resolve submodules with `skip_children=false` and `ignore_missing_submod=false`
    let args = [
        "--config=skip_children=false,ignore_missing_submod=false",
        "tests/source/issue-5609.rs"
    ];
    let (_stdout, stderr) = rustfmt(&args);
    // Module resolution fails because we're unable to find `missing_submod.rs`
    assert!(stderr.contains("missing_submod.rs does not exist"))
}

#[test]
fn ignore_missing_sub_mod_true() {
    // Ensure that missing submodules don't cause module not found errors when trying to
    // resolve submodules with `skip_children=false` and `ignore_missing_submod=true`.
    let args = [
        "--emit=stdout",
        "--config=skip_children=false,ignore_missing_submod=true",
        "tests/source/issue-5609.rs"
    ];
    let (stdout, _stderr) = rustfmt(&args);
    let target = read_to_string(PathBuf::from("tests/target/issue-5609.rs")).unwrap();
    assert!(stdout.ends_with(&target));
}

The more important of the two is ignore_missing_sub_mod_false to explicitly test the behavior when the new ignore_missing_sub option is set to false, but having the ignore_missing_sub_mod_true case right next to it serves as an inline contrast that mimics the system test that you added in tests/target/issue-5609.rs.

[tanzaku]

Thank you for your review. All fixed.

[ytmimi]

@tanzaku I know this one has been left on the backlog for a little while. I wanted to check back in and see if you were still interested in working on it.

[tanzaku]

Yes, I am still interested in working on it.

[ytmimi]

Awesome! I wanted to confirm that before jumping back in. When you have a chance, can you rebase on the latest master and fix any merge conflicts.

[tanzaku]

@ytmimi Sorry, I forgot to post earlier. I've rebased and resolved the conflicts.

[ytmimi]

Thank you, I'll give this one another review later today!

[ytmimi]

Thanks for pushing those changes. Just wanted to follow up and say that I'm planning to look at this one again later this week!