Simplify use crate::rustc_foo::bar occurrences.

They can just be written as `use rustc_foo::bar`, which is far more
standard. (I didn't even know that a `crate::` prefix was valid.)

Author: nnethercote

src/shims/x86/avx2.rs

@@ -1,5 +1,5 @@
-use crate::rustc_middle::ty::layout::LayoutOf as _;
 use rustc_middle::mir;
+use rustc_middle::ty::layout::LayoutOf as _;
 use rustc_middle::ty::Ty;
 use rustc_span::Symbol;
 use rustc_target::spec::abi::Abi;