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_lint/src/passes.rs

@@ -136,7 +136,7 @@ macro_rules! early_lint_methods {
     ($macro:path, $args:tt) => (
         $macro!($args, [
             fn check_param(a: &rustc_ast::Param);
-            fn check_ident(a: &rustc_span::symbol::Ident);
+            fn check_ident(a: &rustc_span::Ident);
             fn check_crate(a: &rustc_ast::Crate);
             fn check_crate_post(a: &rustc_ast::Crate);
             fn check_item(a: &rustc_ast::Item);