rustc_lint_defs: Eliminate the dependency on rustc_hir for Namespace

`rustc_lint_defs` uses `rustc_hir` solely for the `Namespace` type,
which it only needs the static description from. Use the static
description directly, to eliminate the dependency on `rustc_hir`.

This reduces a long dependency chain:
- Many things depend on `rustc_errors`
- `rustc_errors` depends on `rustc_lint_defs`
- `rustc_lint_defs` depended on `rustc_hir` prior to this commit
- `rustc_hir` depends on `rustc_target`

compiler/rustc_lint/src/lints.rs

@@ -8,7 +8,6 @@ use rustc_errors::{
     EmissionGuarantee, LintDiagnostic, MultiSpan, Subdiagnostic, SuggestionStyle,
 };
 use rustc_hir as hir;
-use rustc_hir::def::Namespace;
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::VisitorExt;
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
@@ -2769,7 +2768,7 @@ pub(crate) struct AbsPathWithModuleSugg {
 pub(crate) struct ProcMacroDeriveResolutionFallback {
     #[label]
     pub span: Span,
-    pub ns: Namespace,
+    pub ns_descr: &'static str,
     pub ident: Ident,
 }