new lint: use_crate_prefix_for_self_imports

[lengyijun]

changelog: [use_crate_prefix_for_self_imports]: new lint

fix #13645

Only check main.rs and lib.rs

Known problem

Duplicate warning

TODO

• rm tests/ui/use_crate_prefix_for_self_imports.rs
• squash commits

Question

Although pass all tests, but if copy lintcheck to /tmp, clippy will report two warnings (expected)

warning: this import is not clear
  --> src/main.rs:42:5
   |
42 | use input::read_crates;
   |     ^^^^^ help: prefix with `crate::`: `crate::input`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#use_crate_prefix_for_self_imports
   = note: `#[warn(clippy::use_crate_prefix_for_self_imports)]` on by default

warning: this import is not clear
  --> src/main.rs:43:5
   |
43 | use output::{ClippyCheckOutput, ClippyWarning, RustcIce};
   |     ^^^^^^ help: prefix with `crate::`: `crate::output`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#use_crate_prefix_for_self_imports

[rustbot]

r? @llogiq

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[llogiq]

While I understand that clearing up possible path confusion is a worthy cause, I'm afraid that introducing the lint as warn-by-default will lead to a lot of noise and broken CI jobs.

I'll discuss this on zulip.