make ApkIndexDependency conform to ApkIndexRequirementRef

dunno if this is necessary for the graph to work but we'll see

Sources/apk/Common/ApkVersionSpecification.swift

@@ -3,22 +3,12 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-enum ApkVersionSpecification: Equatable {
+enum ApkVersionSpecification: Equatable, Hashable {
   case any
   case constraint(op: Operator, version: String)
   case conflict
 }
 
-extension ApkVersionSpecification: CustomStringConvertible {
-  var description: String {
-    switch self {
-    case .any: ""
-    case .conflict: "!"
-    case .constraint(let op, let version): "\(op)\(version)"
-    }
-  }
-}
-
 extension ApkVersionSpecification {
   enum Operator: Equatable {
     case equals