Copy link values from template to candidate's ireq

We need to set the original link so the value is accessible in later
code, when we return the ireq backing the candidate. This is needed for
some parts of the post processing like PEP 610 support, which needs to
record the original, remote URL, not the potentially hit cache link.

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

@@ -57,8 +57,8 @@ def make_install_req_from_link(link, template):
             hashes=template.hash_options
         ),
     )
-    if ireq.link is None:
-        ireq.link = link
+    ireq.original_link = template.original_link
+    ireq.link = link
     return ireq