If `True` is not in scope and is used, suggest the valid `true`

[Rageking8]

Given the following code: link

fn main() {
    let x = True;
}

The current output is:

Compiling playground v0.0.1 (/playground)
error[[E0425]](https://doc.rust-lang.org/nightly/error-index.html#E0425): cannot find value `True` in this scope
 --> src/main.rs:2:13
  |
2 |     let x = True;
  |             ^^^^ not found in this scope

For more information about this error, try `rustc --explain E0425`.
error: could not compile `playground` due to previous error

Ideally the output should suggest something like "if this is meant to be a boolean, consider changing it to true", since True is used for other languages.

[vincenzopalazzo]

@rustbot claim