Make a proper get_install_requirement method for candidates

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

@@ -46,3 +46,7 @@ class Candidate(object):
     def get_dependencies(self):
         # type: () -> Sequence[InstallRequirement]
         raise NotImplementedError("Override in subclass")
+
+    def get_install_requirement(self):
+        # type: () -> InstallRequirement
+        raise NotImplementedError("Override in subclass")

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

@@ -124,3 +124,7 @@ class LinkCandidate(Candidate):
             self._make_install_req(str(r), self._ireq)
             for r in self.dist.requires()
         ]
+
+    def get_install_requirement(self):
+        # type: () -> InstallRequirement
+        return self._ireq

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

@@ -38,15 +38,7 @@ class PipProvider(AbstractProvider):
 
     def get_install_requirement(self, c):
         # type: (Candidate) -> InstallRequirement
-
-        # The base Candidate class does not have an _ireq attribute, so we
-        # fetch it dynamically here, to satisfy mypy. In practice, though, we
-        # only ever deal with LinkedCandidate objects at the moment, which do
-        # have an _ireq attribute.  When we have a candidate type for installed
-        # requirements we should probably review this.
-        #
-        # TODO: Longer term, make a proper interface for this on the candidate.
-        return getattr(c, "_ireq", None)
+        return c.get_install_requirement()
 
     def identify(self, dependency):
         # type: (Union[Requirement, Candidate]) -> str