[mejrs]

Implements part of the register_tool rfc, RFC 3808:

Today, #![no_implicit_prelude] suppresses the tool attribute prelude but does not suppress the lint prelude (e.g., #[allow(clippy::foo)] continues to work). Under this RFC, neither prelude is suppressed by #![no_implicit_prelude]. This is technically a breaking change since it can produce new ambiguity errors in stable code.

(The breaking change is already taken care of in #162597, the ambiguity error is raised regardless of whether the no_implicit_prelude attribute is used)

Currently, the following code errors. After this PR, it will start compiling.

#![no_implicit_prelude]
#![feature(register_tool)]
#![register_tool(my_tool)]

#[my_tool::attr] // ERROR cannot find module or crate `my_tool` in this scope
#[rustfmt::skip] // ERROR cannot find module or crate `rustfmt` in this scope
struct X;

Reference PR: rust-lang/reference#2404

r? petrochenkov