Workround for issue #729 (#731)

Workround for issue #729 (#731)

Updated rules in 3.10.2 and 6.4.1(6) caused the use of
Constant_Reference, which takes the Container as an aliased parameter,
to trip over its use on a container passed as an un-aliased parameter.

  * src/alire/alire-conditional_trees.ads (Root): replace
      Constant_Reference by Element.
  * src/alire/alire-dependencies-states.adb ("="): likewise.
  * src/alire/alire-conditional_trees.adb (To_YAML (1)): likewise.
    (Image_One_Line): likewise.
    (Iterate): replace Constant_Reference.Element.all by just Element.