internal/core/adt: fix Vertex dereferencing for disjunctions

When creating disjuncts, the Vertex values created
so far are copied for each disjunct. We must take
this into account when copying over Conjuncts as
well: their Environments may still point to original
nodes, rather than the copied ones, which may lead
to incomplete data.

We originally replaced Environments only
superficially. This mostly worked. However, we
forgot to also update the Environment for tasks.
A straightforward update, however, seems to fix
the outstanding bugs, but introduces some new
ones.

We now take a more complete approach where every
Environment is recursively updated with any new
Vertex.

Here are a few key properties of the new approach:
- We track all Vertex values that are copied and
  make no assumptions that some of them will never
  appear in Environment values.
- When unifying multiple disjunctions, we carry
  over the full Vertex map, to be sure we have a
  complete dereference graph.
- We replace the Vertex values only after all
  Vertex values have been cloned. This prevents us
  from accidentally missing some replacements.

Fixes #3850
Fixes #3875

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I20102183c4ed1e903157909a739d164a8a64ce9b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1213570
Reviewed-by: Matthew Sackman <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mpvl