Minimize pub usage in source_map.rs.

Most notably, this commit changes the `pub use crate::*;` in that file
to `use crate::*;`. This requires a lot of `use` items in other crates
to be adjusted, because everything defined within `rustc_span::*` was
also available via `rustc_span::source_map::*`, which is bizarre.

The commit also removes `SourceMap::span_to_relative_line_string`, which
is unused.

compiler/rustc_parse/src/parser/ty.rs

@@ -18,9 +18,8 @@ use rustc_ast::{
     TraitObjectSyntax, Ty, TyKind,
 };
 use rustc_errors::{Applicability, PResult};
-use rustc_span::source_map::Span;
 use rustc_span::symbol::{kw, sym, Ident};
-use rustc_span::Symbol;
+use rustc_span::{Span, Symbol};
 use thin_vec::{thin_vec, ThinVec};
 
 /// Any `?`, `!`, or `~const` modifiers that appear at the start of a bound.