Avoid rustc_span:: qualifiers.

In several files they are entirely unnecessary, with the relevant names
already imported. And in a few I have added the necessary `use` item.

compiler/rustc_attr_parsing/src/attributes/stability.rs

@@ -117,7 +117,7 @@ impl AttributeParser for BodyStabilityParser {
 pub(crate) struct ConstStabilityIndirectParser;
 // FIXME(jdonszelmann): single word attribute group when we have these
 impl SingleAttributeParser for ConstStabilityIndirectParser {
-    const PATH: &'static [rustc_span::Symbol] = &[sym::rustc_const_stable_indirect];
+    const PATH: &'static [Symbol] = &[sym::rustc_const_stable_indirect];
 
     // ignore
     fn on_duplicate(_cx: &AcceptContext<'_>, _first_span: Span) {}