Associate PackageUnit with ModuleDecl
* Weakly reference ModuleDecl from PackageUnit
* Add PackageUnit decl context getter and use it for a package AccessScope
* Return module decl referenced by PackageUnit in getModuleScopeContext and getParentModule
* Handle package acl in access scope checkers
* Remove AccessLimitKind
* Fix tests
Resolves rdar://104987295, rdar://105187216
