Add 'package' access modifier contextual keyword

Resolves rdar://103462581

Author: elsh

gyb_syntax_support/AttributeKinds.py

@@ -820,6 +820,7 @@ def __init__(self, name, swift_name=None):
     DeclAttributeAlias('fileprivate', 'AccessControl', swift_name='fileprivateKeyword'),
     DeclAttributeAlias('internal', 'AccessControl', swift_name='internalKeyword'),
     DeclAttributeAlias('public', 'AccessControl', swift_name='publicKeyword'),
+    ContextualDeclAttributeAlias('package', 'AccessControl'),
     ContextualDeclAttributeAlias('open', 'AccessControl'),
     DeclAttribute('__setter_access', 'SetterAccess',
                   OnVar,  OnSubscript,