Perform imports from thiserror through absolute path

[dtolnay]

The previous way this was written could have its meaning changed by the presence of a module or type called thiserror, which is different from the crate thiserror. The new expansion refers unambiguously to a crate.

Before:

#[derive(::thiserror::Error, Debug)]
#[error("{0}")]
pub struct Error(i32);

mod thiserror {}

error[E0432]: unresolved import `thiserror`
 --> src/main.rs:1:10
  |
1 | #[derive(::thiserror::Error, Debug)]
  |          ^^^^^^^^^^^^^^^^^^ could not find `__private` in `thiserror`
  |
  = note: this error originates in the derive macro `::thiserror::Error` (in Nightly builds, run with -Z macro-backtrace for more info)

After: compiles successfully.