Re-export more rustc_span::symbol things from rustc_span.

`rustc_span::symbol` defines some things that are re-exported from
`rustc_span`, such as `Symbol` and `sym`. But it doesn't re-export some
closely related things such as `Ident` and `kw`. So you can do `use
rustc_span::{Symbol, sym}` but you have to do `use
rustc_span::symbol::{Ident, kw}`, which is inconsistent for no good
reason.

This commit re-exports `Ident`, `kw`, and `MacroRulesNormalizedIdent`,
and changes many `rustc_span::symbol::` qualifiers in `compiler/` to
`rustc_span::`. This is a 200+ net line of code reduction, mostly
because many files with two `use rustc_span` items can be reduced to
one.

compiler/rustc_feature/src/accepted.rs

@@ -1,6 +1,6 @@
 //! List of the accepted feature gates.
 
-use rustc_span::symbol::sym;
+use rustc_span::sym;
 
 use super::{Feature, to_nonzero};
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -6,7 +6,7 @@ use AttributeDuplicates::*;
 use AttributeGate::*;
 use AttributeType::*;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_span::symbol::{Symbol, sym};
+use rustc_span::{Symbol, sym};
 
 use crate::{Features, Stability};
 

compiler/rustc_feature/src/lib.rs

@@ -28,7 +28,7 @@ mod tests;
 
 use std::num::NonZero;
 
-use rustc_span::symbol::Symbol;
+use rustc_span::Symbol;
 
 #[derive(Debug, Clone)]
 pub struct Feature {

compiler/rustc_feature/src/removed.rs

@@ -1,6 +1,6 @@
 //! List of the removed feature gates.
 
-use rustc_span::symbol::sym;
+use rustc_span::sym;
 
 use super::{Feature, to_nonzero};
 

compiler/rustc_feature/src/unstable.rs

@@ -3,8 +3,7 @@
 use std::path::PathBuf;
 
 use rustc_data_structures::fx::FxHashSet;
-use rustc_span::Span;
-use rustc_span::symbol::{Symbol, sym};
+use rustc_span::{Span, Symbol, sym};
 
 use super::{Feature, to_nonzero};