Add ability to define templateAttributes within a target

This allows parameterizing templates. Also change
placeholder syntax to `${placeholderName}` also for
existing placeholders `$target_name`and `$platform`
and generate warnings when using the old placeholder
syntax.

Sources/XcodeGenKit/SpecLoader.swift

@@ -26,6 +26,10 @@ public class SpecLoader {
         return project
     }
 
+    public func validateProjectDictionaryWarnings() throws {
+        try projectDictionary?.validateWarnings()
+    }
+
     public func generateCacheFile() throws -> CacheFile? {
         guard let projectDictionary = projectDictionary,
             let project = project else {
@@ -38,3 +42,36 @@ public class SpecLoader {
         )
     }
 }
+
+private extension Dictionary where Key == String, Value: Any {
+
+    func validateWarnings() throws {
+        var errors: [SpecValidationError.ValidationError] = []
+        if hasValueContaining("$target_name") {
+            errors.append(.deprecatedUsageOfPlaceholder(placeholderName: "target_name"))
+        }
+        if hasValueContaining("$platform") {
+            errors.append(.deprecatedUsageOfPlaceholder(placeholderName: "platform"))
+        }
+        if !errors.isEmpty {
+            throw SpecValidationError(errors: errors)
+        }
+    }
+
+    func hasValueContaining(_ needle: String) -> Bool {
+        return values.contains { value in
+            switch value {
+            case let dictionary as JSONDictionary:
+                return dictionary.hasValueContaining(needle)
+            case let string as String:
+                return string.contains(needle)
+            case let array as [JSONDictionary]:
+                return array.contains { $0.hasValueContaining(needle) }
+            case let array as [String]:
+                return array.contains { $0.contains(needle) }
+            default:
+                return false
+            }
+        }
+    }
+}