Allow reference to polymorphic variants when possible

[panglesd]

A break from the 3.0 design!

Fixes #907 by allowing references to polymorphic variants when the type is given in the reference and the type's manifest is a polymorphic variant.

The ` in the constructor name is allowed and not mandatory.

References without specifying the parent type were not requested in #907 and could be added in another PR.

The last commits illustrate a problem: the polymorphic variants do not have an ID, so the resolved references for polymorphic variants are turned into a normal constructor identifier. This is not ideal. If needed, the model could be modified to represent for "types whose manifest is a single polymorphic variant" in a way that allows to define ids to the polymorphic variants. That is a bigger refactoring, but I can do it if you think that is needed.

type switch = [ `On | `Off ]

(**

   References with type as parent work:
   - {!type-switch.On}
   - {!type-switch.`Off}
   - {!type-switch.constructor-On}
   - {!type-switch.constructor-`Off}
   - {!switch.On}
   - {!switch.`Off}
   - {!switch.constructor-On}
   - {!switch.constructor-`Off}

   References in the environment don't work:
   - {!On}
   - {!`On}
   - {!constructor-On}
   - {!constructor-`On}

*)