Simplify marker simplification (#530)

Try to use reduction to DNF and CNF as much as possible to achieve marker simplifications, in the hope that:

   - it's more general than the collection of heuristics we currently have
   - it simplifies the code

The potential downside of this approach is that cnf() and dnf() can,
in the worst case, be exponentially expensive.

eg `cnf((a1 and b1) or (a2 and b2) or ... (an and bn))` generates 2^n
intersections.

In practice, markers on python packages seem not to get long enough for
this to be an issue.

Co-authored-by: Randy Döring <30527984+radoering@users.noreply.github.com>

src/poetry/core/packages/utils/utils.py

@@ -179,8 +179,8 @@ def add_constraint(
  for i, sub_marker in enumerate(conjunctions):
  if isinstance(sub_marker, MultiMarker):
  for m in sub_marker.markers:
- if isinstance(m, SingleMarker):
-  add_constraint(m.name, (m.operator, m.value), i)
+ assert isinstance(m, SingleMarker)
+ add_constraint(m.name, (m.operator, m.value), i)
  elif isinstance(sub_marker, SingleMarker):
  add_constraint(sub_marker.name, (sub_marker.operator, sub_marker.value), i)