Convert target path from Path to str & PurePath

The target path is just read as string from the CLI now, to allow more
complex targets (such as URLs) that don't directly map to a path.

The Packer subclass now handles the conversion from that string to a
`pathlib.PurePath`, and specific subclasses & transfer classes can convert
those to a `pathlib.Path` to perform actual filesystem operations when
necessary.

blender_asset_tracer/pack/s3.py

@@ -120,7 +120,7 @@ class S3Transferrer(transfer.FileTransferer):
         if files_skipped:
             log.info('Skipped %d files', files_skipped)
 
-    def upload_file(self, src: pathlib.Path, dst: pathlib.Path) -> bool:
+    def upload_file(self, src: pathlib.Path, dst: pathlib.PurePath) -> bool:
         """Upload a file to an S3 bucket.
 
         The first part of 'dst' is used as the bucket name, the remained as the