Ensure requirements are prepared before get_dist()

The legacy resolver seems to prepare requirements differently that can
leave them half-done in certain code paths. I'm not quite sure how it's
doing things, but at least we can do this to make sure they do things
right.

src/pip/_internal/commands/download.py

@@ -130,7 +130,6 @@ class DownloadCommand(RequirementCommand):
         self.trace_basic_info(finder)
 
         requirement_set = resolver.resolve(reqs, check_supported_wheels=True)
-        requirement_set.warn_legacy_versions_and_specifiers()
 
         downloaded: List[str] = []
         for req in requirement_set.requirements.values():
@@ -138,6 +137,10 @@ class DownloadCommand(RequirementCommand):
                 assert req.name is not None
                 preparer.save_linked_requirement(req)
                 downloaded.append(req.name)
+
+        preparer.prepare_linked_requirements_more(requirement_set.requirements.values())
+        requirement_set.warn_legacy_versions_and_specifiers()
+
         if downloaded:
             write_output("Successfully downloaded %s", " ".join(downloaded))
 

src/pip/_internal/commands/wheel.py

@@ -145,7 +145,6 @@ class WheelCommand(RequirementCommand):
         self.trace_basic_info(finder)
 
         requirement_set = resolver.resolve(reqs, check_supported_wheels=True)
-        requirement_set.warn_legacy_versions_and_specifiers()
 
         reqs_to_build: List[InstallRequirement] = []
         for req in requirement_set.requirements.values():
@@ -154,6 +153,9 @@ class WheelCommand(RequirementCommand):
             elif should_build_for_wheel_command(req):
                 reqs_to_build.append(req)
 
+        preparer.prepare_linked_requirements_more(requirement_set.requirements.values())
+        requirement_set.warn_legacy_versions_and_specifiers()
+
         # build wheels
         build_successes, build_failures = build(
             reqs_to_build,

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

@@ -159,6 +159,9 @@ class Resolver(BaseResolver):
 
         reqs = req_set.all_requirements
         self.factory.preparer.prepare_linked_requirements_more(reqs)
+        for req in reqs:
+            req.prepared = True
+            req.needs_more_preparation = False
         return req_set
 
     def get_installation_order(