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_abi/src/extern_abi/mod.rs

@@ -1,8 +1,7 @@
 use std::fmt;
 
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
-use rustc_span::symbol::sym;
-use rustc_span::{Span, Symbol};
+use rustc_span::{Span, Symbol, sym};
 
 #[cfg(test)]
 mod tests;