Create PackageUnit class, and Package entries to DeclContext / ASTHierarchy

Previously enum AccessLimitKind was
added to distinguish access scopes b/t package and public while keeping
DeclContext null but it proved to be too limiting. This PR creates package specific entries for DeclContext and
ASTHierarchy. It create a new class PackageUnit that can be set as the parent DeclContext of ModuleDecl. This PR
contains addition of such entries but not the use of them; the actual use of them will be in the upcoming PRs.

Resolves rdar://106155600

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -1198,6 +1198,8 @@ static Optional<AccessLevel> inferAccessSyntactically(const ValueDecl *D) {
   case DeclContextKind::AbstractFunctionDecl:
   case DeclContextKind::SubscriptDecl:
     return AccessLevel::Private;
+  case DeclContextKind::Package:
+    return AccessLevel::Package;
   case DeclContextKind::Module:
   case DeclContextKind::FileUnit:
   case DeclContextKind::MacroDecl: