Move download file copying out of unpacking functions

Now our "unpacking" functions aren't also for sometimes populating
the download directory.

src/pip/_internal/operations/prepare.py

@@ -165,12 +165,6 @@ def unpack_http_url(
     # downloading archives, they have to be unpacked to parse dependencies
     unpack_file(from_path, location, content_type)
 
-    # a download dir is specified; let's copy the archive there
-    if download_dir and not os.path.exists(
-        os.path.join(download_dir, link.filename)
-    ):
-        _copy_file(from_path, download_dir, link)
-
     return from_path
 
 
@@ -261,12 +255,6 @@ def unpack_file_url(
     # archives, they have to be unpacked to parse dependencies
     unpack_file(from_path, location, content_type)
 
-    # a download dir is specified and not already downloaded
-    if download_dir and not os.path.exists(
-        os.path.join(download_dir, link.filename)
-    ):
-        _copy_file(from_path, download_dir, link)
-
     return from_path
 
 
@@ -545,6 +533,10 @@ def prepare_linked_requirement(
             if download_dir:
                 if link.is_existing_dir():
                     logger.info('Link is a directory, ignoring download_dir')
+                elif local_path and not os.path.exists(
+                    os.path.join(download_dir, link.filename)
+                ):
+                    _copy_file(local_path, download_dir, link)
 
             if self._download_should_save:
                 # Make a .zip of the source_dir we already created.