Better completion with module alias and use directives

[soutaro]

Type name completion didn't work with module/class aliases or use directives

use Foo as Foo2

module Foo
  module Bar
  end
end

module Foo1 = Foo

type t1 = Foo1::B    # <= Completion didn't work because `Foo1` is a module alias
type t2 = Foo2::B   # <= Completion didn't work because `Foo2` is a `use` type name

This also fixes a completion triggered with an absolute type name -- ::Foo.