Implement extra-ed requirement merging

When a requirement is requested multiple times, some via a direct URL
("req @ URL") and some not but with extras ("req[extra] VERSION"), the
resolver previous could not correctly find "req[extra]" if "req" is
available in an index.

This additional logic makes the resolver, when encountering a
requirement with identifier "req[extra]", to also look for explicit
candidates listed under "req", and add them as found matches for
"req[extra]".

news/8785.bugfix.rst

@@ -0,0 +1,4 @@
+New resolver: When a requirement is requested both via a direct URL
+(``req @ URL``) and via version specifier with extras (``req[extra]``), the
+resolver will now be able to use the URL to correctly resolve the requirement
+with extras.

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

@@ -16,6 +16,10 @@
     cast,
 )
 
+from pip._vendor.packaging.requirements import (
+    InvalidRequirement,
+    Requirement as PackagingRequirement,
+)
 from pip._vendor.packaging.specifiers import SpecifierSet
 from pip._vendor.packaging.utils import NormalizedName, canonicalize_name
 from pip._vendor.pkg_resources import Distribution
@@ -296,6 +300,24 @@ def find_candidates(
             if ireq is not None:
                 ireqs.append(ireq)
 
+        # If the current identifier contains extras, also add explicit
+        # candidates from entries from extra-less identifier.
+        try:
+            identifier_req = PackagingRequirement(identifier)
+        except InvalidRequirement:
+            base_identifier = None
+            extras = frozenset()
+        else:
+            base_identifier = identifier_req.name
+            extras = frozenset(identifier_req.extras)
+        if base_identifier and base_identifier in requirements:
+            for req in requirements[base_identifier]:
+                base_cand, _ = req.get_candidate_lookup()
+                if base_cand is None:
+                    continue
+                candidate = self._make_extras_candidate(base_cand, extras)
+                explicit_candidates.add(candidate)
+
         # If none of the requirements want an explicit candidate, we can ask
         # the finder for candidates.
         if not explicit_candidates: