add package validation

Sources/ProjectSpec/SpecValidation.swift

@@ -156,6 +156,10 @@ extension Project {
                             errors.append(.invalidSDKDependency(target: target.name, dependency: dependency.reference))
                         }
                     }
+                case .package:
+                    if packages[dependency.reference] == nil {
+                        errors.append(.invalidSwiftPackage(name: dependency.reference, target: target.name))
+                    }
                 default: break
                 }
             }

Sources/ProjectSpec/SpecValidationError.swift

@@ -18,6 +18,7 @@ public struct SpecValidationError: Error, CustomStringConvertible {
         case invalidTargetSchemeTest(target: String, testTarget: String)
         case invalidSchemeTarget(scheme: String, target: String)
         case invalidSchemeConfig(scheme: String, config: String)
+        case invalidSwiftPackage(name: String, target: String)
         case invalidConfigFile(configFile: String, config: String)
         case invalidBuildSettingConfig(String)
         case invalidSettingsGroup(String)
@@ -61,6 +62,8 @@ public struct SpecValidationError: Error, CustomStringConvertible {
                 return "Invalid file group \(group.quoted)"
             case let .invalidConfigFileConfig(config):
                 return "Config file has invalid config \(config.quoted)"
+            case let .invalidSwiftPackage(name, target):
+                return "Target \(target.quoted) has an invalid package dependency \(name.quoted)"
             case let .missingConfigForTargetScheme(target, configType):
                 return "Target \(target.quoted) is missing a config of type \(configType.rawValue) to generate its scheme"
             case let .missingDefaultConfig(name):