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_parse/src/lexer/mod.rs

@@ -14,8 +14,7 @@ use rustc_session::lint::builtin::{
     TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
 };
 use rustc_session::parse::ParseSess;
-use rustc_span::symbol::Symbol;
-use rustc_span::{BytePos, Pos, Span};
+use rustc_span::{BytePos, Pos, Span, Symbol};
 use tracing::debug;
 
 use crate::lexer::diagnostics::TokenTreeDiagInfo;

compiler/rustc_parse/src/lexer/unicode_chars.rs

@@ -1,8 +1,7 @@
 //! Characters and their corresponding confusables were collected from
 //! <https://www.unicode.org/Public/security/10.0.0/confusables.txt>
 
-use rustc_span::symbol::kw;
-use rustc_span::{BytePos, Pos, Span};
+use rustc_span::{BytePos, Pos, Span, kw};
 
 use super::Lexer;
 use crate::errors::TokenSubstitution;