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

@@ -136,6 +136,7 @@ def run(self, options, args):
 
         downloaded = []  # type: List[str]
         for req in requirement_set.requirements.values():
+            preparer.save_linked_requirement(req)
             if not req.editable and req.satisfied_by is None:
                 assert req.name is not None
                 downloaded.append(req.name)

src/pip/_internal/commands/wheel.py

@@ -21,6 +21,8 @@
     from optparse import Values
     from typing import List
 
+    from pip._internal.req.req_install import InstallRequirement
+
 
 logger = logging.getLogger(__name__)
 
@@ -156,10 +158,12 @@ 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

@@ -547,24 +547,29 @@ def _prepare_linked_requirement(self, req, parallel_builds):
             dist = _get_prepared_distribution(
                 req, self.req_tracker, self.finder, self.build_isolation,
             )
+        return dist
 
-            if download_dir:
-                if link.is_existing_dir():
-                    logger.info('Link is a directory, ignoring download_dir')
-                elif local_file:
-                    download_location = os.path.join(
-                        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 req.link
+        link = req.link
+        if self._download_should_save and link.is_vcs:
+            # Make a .zip of the source_dir we already created.
+            req.archive(self.download_dir)
 
-            if self._download_should_save:
-                # Make a .zip of the source_dir we already created.
-                if link.is_vcs:
-                    req.archive(self.download_dir)
-        return dist
+        if link.is_existing_dir():
+            logger.debug(
+                'Not copying link to destination directory '
+                'since it is a directory: %s', link,
+            )
+        elif req.local_file_path is not None:
+            download_dir = self._get_download_dir(link)
+            assert download_dir is not None
+            download_location = os.path.join(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,