[entropy.client.interfaces.dep] SLOT intersection: alter dependency adding slot

Scenario:
dependency = dev-lang/python
SLOT intersection = enabled
available slots = 2.6, 2.7, 3.2
installed slot = 2.7

The SLOT intersection feature worked correctly but in the end
dev-lang/python was added to the unsatisfied set because there
was a dev-lang/python-2.7 update as well. This results in other
dep calculation code to match dev-lang/python against 3.2 instead.
The SLOT intersection feature has to reduce the dependency string
scope by adding a SLOT suffix (in this case: ":2.7"). This way the
correct dependency is eventually pulled in.

lib/entropy/client/interfaces/dep.py

@@ -688,10 +688,18 @@ class CalculatorsMixin:
                 if installed_slot in available_slots:
                     # restrict my matching to installed_slot, rewrite
                     # r_id r_repo
+                    # NOTE: assume that dependency has no tag nor etp rev
+                    # also, if we got multiple slots, it means that the
+                    # same dep is expressed without slot.
                     old_r_id = r_id
                     old_r_repo = r_repo
                     r_id, r_repo = self.atom_match(
                         dependency, match_slot = installed_slot)
+                    if r_id != -1:
+                        # append slot to dependency
+                        dependency += etpConst['entropyslotprefix'] \
+                            + installed_slot
+
                     if const_debug_enabled():
                         from_atom = self.open_repository(
                             old_r_repo).retrieveAtom(old_r_id)