Merge spec->req constructors

Since both functions now return Optional[Requirement], we can just use
the same implementation.

Author: uranusjr

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

@@ -445,7 +445,7 @@ def iter_dependencies(self):
         yield factory.make_requirement_from_candidate(self.base)
 
         for r in self.base.dist.requires(valid_extras):
-            requirement = factory.make_requirement_from_spec_matching_extras(
+            requirement = factory.make_requirement_from_spec(
                 str(r), self.base._ireq, valid_extras,
             )
             if requirement:

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

@@ -256,12 +256,7 @@ def make_requirement_from_candidate(self, candidate):
         # type: (Candidate) -> ExplicitRequirement
         return ExplicitRequirement(candidate)
 
-    def make_requirement_from_spec(self, specifier, comes_from):
-        # type: (str, InstallRequirement) -> Requirement
-        ireq = self._make_install_req_from_spec(specifier, comes_from)
-        return self.make_requirement_from_install_req(ireq, ())
-
-    def make_requirement_from_spec_matching_extras(
+    def make_requirement_from_spec(
         self,
         specifier,  # type: str
         comes_from,  # type: InstallRequirement