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`

Cargo.lock

@@ -4139,7 +4139,6 @@ dependencies = [
  "rustc_ast",
  "rustc_data_structures",
  "rustc_error_messages",
- "rustc_hir",
  "rustc_hir_id",
  "rustc_macros",
  "rustc_serialize",