Move WordPressComRestApiErrorDomain to APIInterface

This DRYs the definition, makes the constant available to both
Objective-C and Swift, and removes the need for custom builder and
checker.

See @crazytonyli's suggestion from
wordpress-mobile/WordPressKit-iOS#782 (comment)

Author: mokagio

WooCommerce/WordPressAuthenticatorTests/WordPressKit/CoreAPITests/WordPressComRestApiTests+Error.swift

@@ -1,5 +1,6 @@
 import XCTest
 #if SWIFT_PACKAGE
+import APIInterface
 @testable import CoreAPI
 #else
 @testable import WordPressKit