Replace already_downloaded_path with existence check

These statements are equivalent, so we exchange them. This will make it
easier to factor the download_dir concerns out of these functions.

src/pip/_internal/operations/prepare.py

@@ -140,10 +140,9 @@ def unpack_http_url(
         unpack_file(from_path, location, content_type)
 
         # a download dir is specified; let's copy the archive there
-        if download_dir and not already_downloaded_path:
-            assert not os.path.exists(
-                os.path.join(download_dir, link.filename)
-            )
+        if download_dir and not os.path.exists(
+            os.path.join(download_dir, link.filename)
+        ):
             _copy_file(from_path, download_dir, link)
 
 
@@ -237,8 +236,9 @@ def unpack_file_url(
     unpack_file(from_path, location, content_type)
 
     # a download dir is specified and not already downloaded
-    if download_dir and not already_downloaded_path:
-        assert not os.path.exists(os.path.join(download_dir, link.filename))
+    if download_dir and not os.path.exists(
+        os.path.join(download_dir, link.filename)
+    ):
         _copy_file(from_path, download_dir, link)