suggest declaring modules when file found but module not defined

[bend-n]

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

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:

= help: you may have forgotten to declare the module `thing`. use `mod thing` in this file to declare this module.

[bend-n]

@rustbot label A-diagnostics