Inlines bodies of library_add_folder, library_cleanup_missing_files,
library_get_folders, and library_get_folders_with_metadata from
library/commands.rs directly into the v2 wrappers in commands_v2/library.rs.
Deletes the corresponding legacy #[tauri::command] definitions and
removes their re-exports from library/mod.rs.
Step 8 of legacy cleanup, group B.
cc