Lazy-evaluate candidates with installed inserted

I don't really know why we didn't do this before, but the implementation
seems to match the description in the docstring.

src/pip/_internal/resolution/resolvelib/found_candidates.py

@@ -1,5 +1,4 @@
 import itertools
-import operator
 
 from pip._vendor.six.moves import collections_abc  # type: ignore
 
@@ -31,19 +30,11 @@ def _insert_installed(installed, others):
     This iterator is used when the resolver prefers to upgrade an
     already-installed package. Candidates from index are returned in their
     normal ordering, except replaced when the version is already installed.
-
-    Since candidates from index are already sorted by reverse version order,
-    `sorted()` here would keep the ordering mostly intact, only shuffling the
-    already-installed candidate into the correct position. We put the already-
-    installed candidate in front of those from the index, so it's put in front
-    after sorting due to Python sorting's stableness guarentee.
     """
-    candidates = sorted(
-        itertools.chain([installed], others),
-        key=operator.attrgetter("version"),
-        reverse=True,
-    )
-    return iter(candidates)
+    for candidate in others:
+        if candidate.version == installed.version:
+            candidate = installed
+        yield candidate
 
 
 class FoundCandidates(collections_abc.Sequence):