Unify DeclaredName and DesignatedName as just Identifier (#2939)

This is just a simplification: I think these different forms are getting in the way more than they're helping, particularly as I was looking into namespace functionality. The handling in semantics can be identical, providing a more uniform behavior.

toolchain/parser/parser_handle_package.cpp

@@ -20,7 +20,7 @@ auto ParserHandlePackage(ParserContext& context) -> void {
   };
 
   if (!context.ConsumeAndAddLeafNodeIf(TokenKind::Identifier,
-                                       ParseNodeKind::DeclaredName)) {
+                                       ParseNodeKind::Identifier)) {
     CARBON_DIAGNOSTIC(ExpectedIdentifierAfterPackage, Error,
                       "Expected identifier after `package`.");
     context.emitter().Emit(*context.position(), ExpectedIdentifierAfterPackage);