Merge #4227

4227: Report invalid, nested, multi-segment crate-paths r=matklad a=djrenren

There was a bug in the previous path-validating code that didn't detect multi-segment paths that started with `crate`.

```rust
// Successfully reported
use foo::{crate};

// BUG: was not being reported
use foo::{crate::bar};
```

This was due to my confusion about path-associativity. That is, the path with no qualifier is the innermost path, not the outermost. I've updated the code with a lot of comments to explain what's going on. 

This bug was discovered when I found an erroneous `ok` test which I reported here: 
https://github.com/rust-analyzer/rust-analyzer/issues/4226

This test now fails and has been modified, hopefully in the spirit of the original test, to be correct.  Sorry about submitting the bug in the first place!

Co-authored-by: John Renner <[email protected]>

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