Vanilla `extend_namespace` no longer works

[merak48763]

This behavior was introduced in e7405ae, which fixed the race condition issue #479.
The commit added an empty temporary resource pack to download objects, which ignores extend_namespace in user's resource pack.

Example code to reproduce the issue

from beet import Context, BinaryFile, NamespaceFileScope
from beet.contrib.vanilla import Vanilla
from typing import ClassVar

class UnihexZip(BinaryFile):
  scope: ClassVar[NamespaceFileScope] = ()
  extension: ClassVar[str] = ".zip"

def beet_default(ctx: Context):
  vanilla = ctx.inject(Vanilla)
  vanilla.assets.extend_namespace.append(UnihexZip)
  unihex = vanilla.mount("assets/minecraft/font", fetch_objects=True).assets[UnihexZip]
  print(unihex)

Expected result: the 3 unihex zip files are listed
Observed result: nothing