New resolver: Avoid polluting dest dir

Previously, during dependency resolution for `pip download -d <dir>`
or `pip wheel -w <dir>`, distributions downloaded are always saved
to <dir>, even for those are only used in backtracking and are not
part of the returned requirement set.

news/8827.bugfix

@@ -0,0 +1,2 @@
+Avoid polluting the destination directory by resolution artifacts
+when the new resolver is used for ``pip download`` or ``pip wheel``.

src/pip/_internal/commands/download.py

@@ -137,6 +137,7 @@ def run(self, options, args):
         for req in requirement_set.requirements.values():
             if not req.editable and req.satisfied_by is None:
                 assert req.name is not None
+                preparer.save_linked_requirement(req)
                 downloaded.append(req.name)
         if downloaded:
             write_output('Successfully downloaded %s', ' '.join(downloaded))

src/pip/_internal/commands/wheel.py

@@ -21,6 +21,7 @@
     from optparse import Values
     from typing import List
 
+    from pip._internal.req.req_install import InstallRequirement
 
 logger = logging.getLogger(__name__)
 
@@ -156,10 +157,12 @@ def run(self, options, args):
             reqs, check_supported_wheels=True
         )
 
-        reqs_to_build = [
-            r for r in requirement_set.requirements.values()
-            if should_build_for_wheel_command(r)
-        ]
+        reqs_to_build = []  # type: List[InstallRequirement]
+        for req in requirement_set.requirements.values():
+            if req.is_wheel:
+                preparer.save_linked_requirement(req)
+            elif should_build_for_wheel_command(req):
+                reqs_to_build.append(req)
 
         # build wheels
         build_successes, build_failures = build(

src/pip/_internal/operations/prepare.py

@@ -526,23 +526,33 @@ def _prepare_linked_requirement(self, req, parallel_builds):
         dist = _get_prepared_distribution(
             req, self.req_tracker, self.finder, self.build_isolation,
         )
+        return dist
 
-        if self.download_dir is not None:
-            if link.is_existing_dir():
-                logger.info('Link is a directory, ignoring download_dir')
-            elif local_file:
-                download_location = os.path.join(
-                    self.download_dir, link.filename
-                )
-                if not os.path.exists(download_location):
-                    shutil.copy(local_file.path, download_location)
-                    download_path = display_path(download_location)
-                    logger.info('Saved %s', download_path)
-
+    def save_linked_requirement(self, req):
+        # type: (InstallRequirement) -> None
+        assert self.download_dir is not None
+        assert req.link is not None
+        link = req.link
         if link.is_vcs:
             # Make a .zip of the source_dir we already created.
             req.archive(self.download_dir)
-        return dist
+            return
+
+        if link.is_existing_dir():
+            logger.debug(
+                'Not copying link to destination directory '
+                'since it is a directory: %s', link,
+            )
+            return
+        if req.local_file_path is None:
+            # No distribution was downloaded for this requirement.
+            return
+
+        download_location = os.path.join(self.download_dir, link.filename)
+        if not os.path.exists(download_location):
+            shutil.copy(req.local_file_path, download_location)
+            download_path = display_path(download_location)
+            logger.info('Saved %s', download_path)
 
     def prepare_editable_requirement(
         self,