refactor: pre-workspaces cleanups

[mkatychev]

Split changes from #231 in preparation for WorkspaceConfig/WorkspaceRootConfig additions.

• added PackageRef to Error::VersionAlreadyExists:
  

  wasm-pkg-tools/crates/wasm-pkg-common/src/lib.rs

  Lines 58 to 59 in 068a2b3

  	#[error("{0}@{1} already exists in the registry")]
  	VersionAlreadyExists(PackageRef, semver::Version),

• introduce pub const WIT_DEPS_DIR:
  

  wasm-pkg-tools/crates/wasm-pkg-core/src/wit.rs

  Lines 31 to 32 in 068a2b3

  	/// Directory holding WIT dependencies for one or more packages
  	pub const WIT_DEPS_DIR: &str = "deps";

• add PublishVerifier to reuse logic between current multi package publishing eventual workspace fetching found in feat(wkg,core,client): intial wkg workspace approach #231:
  

  wasm-pkg-tools/crates/wkg/src/overlay.rs

  Lines 21 to 31 in 790b44c

  	/// A [`CachingClient`] and [`PublishPlan`] wired to a temporary local backend
  	pub(crate) struct PublishVerifier {
  	#[expect(dead_code, reason = "workspaces")]
  	pub(crate) client: CachingClient<FileCache>,
  	pub(crate) plan: PublishPlan,
  	#[expect(dead_code, reason = "workspaces")]
  	pub(crate) packages: BTreeSet<PackageRef>,
  	pub(crate) data: HashMap<PackageRef, Vec<u8>>,
  	/// Held so the temp local backend outlives the returned client.
  	_local_config: LocalConfig,
  	}

[vados-cosmonic]

LGTM 🚀