We should move Rule, Ruleset and related classes into nspmd.lang.rule.

Types that could also be moved there include:

• Rule
• RuleSetFactory (package private)
• nspmd.rules.RuleFactory (package private)
• RuleSetFactoryCompatibility (package private)
• RuleSetLoader
• RuleSet
• RuleSets
• RulePriority
• RuleSetLoadException
• RuleSetWriter
• RuleSetReferenceId

The purpose of this is to clean up the root nspmd package which is currently a mess.

(Part of #3916)