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_ast_lowering/src/lib.rs

@@ -71,9 +71,8 @@ use rustc_middle::{
 };
 use rustc_session::parse::{add_feature_diagnostics, feature_err};
 use rustc_span::hygiene::MacroKind;
-use rustc_span::source_map::DesugaringKind;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::{DesugaringKind, Span, DUMMY_SP};
 use smallvec::SmallVec;
 use std::collections::hash_map::Entry;
 use thin_vec::ThinVec;