refactor: canonicalize moved paths that refer to item left behind

[ahomescu]

Follow-up limitation found while writing the test — FIXED: items moved out
of a header could not rely on bindings that stay behind in the header (e.g.
the kept glob). A bare path like thing that resolved through the glob was
not canonicalized on the way out (is_relative_path treats only
self::/super:: as relative), so the moved item failed to compile with
E0412. move_items now runs canonicalize_moved_decl_paths over every
moved declaration (and its impl block): a single-segment path to a local,
non-moving item def is rewritten to its absolute path via def_qpath.
Paths are left alone when the destination module still binds the ident to
the same def — through a pre-existing import, an import moving along with
the declaration (moved use targets are now recorded in
ModuleInfo::import_targets, mirroring update_module_info_items), or
because the target is defined in the destination module itself — which
keeps the existing snapshots byte-identical. Paths to moved defs are still
rewritten later by update_paths via path_mapping. The
reorganize_glob_import test now uses the bare thing field type and
verifies it becomes crate::defs::thing.

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