Ensure root requirement name is canonicalised

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

@@ -169,7 +169,7 @@ class Factory(object):
     def make_requirement_from_install_req(self, ireq, root=False):
         # type: (InstallRequirement, bool) -> Requirement
         if root and ireq.name:
-            self.root_reqs.add(ireq.name)
+            self.root_reqs.add(canonicalize_name(ireq.name))
 
         if ireq.link:
             # TODO: Get name and version from ireq, if possible?

tests/functional/test_new_resolver.py

@@ -561,7 +561,7 @@ def test_new_resolver_upgrade_needs_option(script):
         "--no-cache-dir", "--no-index",
         "--find-links", script.scratch_path,
         "--upgrade",
-        "pkg",
+        "PKG",  # Deliberately uppercase to check canonicalization
     )
 
     assert "Uninstalling pkg-1.0.0" in result.stdout, str(result)