Prepare lazy wheels more so they are downloaded

This keeps all knowledge about preparation and types of requirements in
`RequirementPreparer`, so there's one place to look when we're ready to
start breaking it apart later.

src/pip/_internal/operations/prepare.py

@@ -486,7 +486,21 @@ class RequirementPreparer(object):
         self._log_preparing_link(req)
         wheel_dist = self._fetch_metadata(link)
         if wheel_dist is not None:
+            req.needs_more_preparation = True
             return wheel_dist
+        return self._prepare_linked_requirement(req, parallel_builds)
+
+    def prepare_linked_requirement_more(self, req, parallel_builds=False):
+        # type: (InstallRequirement, bool) -> None
+        """Prepare a linked requirement more, if needed."""
+        if not req.needs_more_preparation:
+            return
+        self._prepare_linked_requirement(req, parallel_builds)
+
+    def _prepare_linked_requirement(self, req, parallel_builds):
+        # type: (InstallRequirement, bool) -> Distribution
+        assert req.link
+        link = req.link
         if link.is_wheel and self.wheel_download_dir:
             # Download wheels to a dedicated dir when doing `pip wheel`.
             download_dir = self.wheel_download_dir

src/pip/_internal/req/req_install.py

@@ -213,6 +213,9 @@ class InstallRequirement(object):
         # but after loading this flag should be treated as read only.
         self.use_pep517 = use_pep517
 
+        # This requirement needs more preparation before it can be built
+        self.needs_more_preparation = False
+
     def __str__(self):
         # type: () -> str
         if self.req:

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

@@ -159,6 +159,9 @@ class Resolver(BaseResolver):
 
             req_set.add_named_requirement(ireq)
 
+        for actual_req in req_set.all_requirements:
+            self.factory.preparer.prepare_linked_requirement_more(actual_req)
+
         return req_set
 
     def get_installation_order(self, req_set):