Allow qualified names in concrete patterns

[PaulKlint]

Based on the discussion in #502, we need to extend the Rascal syntax to permit qualified names in the symbol that types a concrete syntax fragment or ConcreteHole.

It concerns the following syntax rules:

lexical Concrete 
  = typed: "(" LAYOUTLIST l1 Sym symbol LAYOUTLIST l2 ")" LAYOUTLIST l3 "`" ConcretePart* parts "`";

syntax ConcreteHole 
  = \one: "\<" Sym symbol Name name "\>"
  ;

[jurgenvinju]

It would be consistent if we can qualify nonterminals as well, but for this case it wouldn't help to solve #502? The symbol in the concrete pattern is guaranteed to be a nonterminal and nonterminals are merged automatically if multiple definitions come in scope.

[mahills]

That's actually a good point -- it doesn't really address #502 but it does address the more general problem of disambiguating type names. We do have a similar issue there -- if we have both a concrete and an abstract version of the same name, their unqualified forms are not brought into scope, meaning that the qualified versions have to be used -- see #493 for this. There are options there as well, since we may be able to automatically disambiguate in cases such as "used inside a concrete syntax pattern".