Add duplicate_mod lint

[Alexendoo]

Inspired by #8827, warns if there's a single file that is loaded by more than one mod item

// lib.rs
mod a;
mod b;

// a.rs
#[path = "./b.rs"]
mod b;

It adds a canonicalize call per mod file; encountered, which I don't think should be too heavy

Integration tests with common modules, e.g. test_utils doesn't trigger it as each test is compiled separately, however I couldn't figure out a good way to add a test for that

changelog: Add [duplicate_mod] lint

[rust-highfive]

r? @Manishearth

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[Manishearth]

r? @xFrednet

I'm back from the mountains but trying to reduce my pile of notifications rn so cutting down on reviews

[Manishearth]

@bors r+

actually never mind, this is a pretty straightforward PR 😄

[bors]

📌 Commit 1f79a44 has been approved by Manishearth

[bors]

⌛ Testing commit 1f79a44 with merge c10bfae...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing c10bfae to master...

[SkyfallWasTaken]

Nice to see that this is finally here!