CLJS-646: single segment namespaces and reify don't work

the exists? call in reify was not correct, it was a property access on
the namespace, which would of course get resolved by the compiler,
creating a symbol foo/foo (in the case where foo is the single segment
namespace). now we actually emit a symbol of the namespace and the type
symbol, the exists? macro now annotates this symbol with
:cljs.analyzer/no-resolve, the analyzer now respects this so the user
won't get spurious warnings from using exists? on vars that don't exist.