Rollup merge of #142730 - bend-n:suggest_declaring_modules_when_file_found_but_module_not_defined, r=petrochenkov

suggest declaring modules when file found but module not defined

suggests declaring modules when a module is found but not defined, i.e
```
├── main.rs: `use thing::thang;`
└── thing.rs: `struct thang`
```
or
```
├── main.rs: `use thing::thang;`
└── thing
    └── mod.rs: `struct thang`
```
which currently is just
```rust
error[E0432]: unresolved import `yeah`
 --> src/main.rs:1:1
  |
1 | use thing::thang;
  |     ^^^^^ use of unresolved module or unlinked crate `thing`
  |
```
but now would have this nice help:
```text
= help: you may have forgotten to declare the module `thing`. use `mod thing` in this file to declare this module.
```

Author: matthiaskrgr