stage: provide mirrors in constructor (#45792)

Stage objects create mirrors ad-hoc from current config.

- There is no way to prevent mirrors from being used
- There is no way to restrict mirrors to source/binary, which is of
  course context dependent.
- Stage is also used in build caches, where iterating over mirrors is
  already implemented differently, and wouldn't work anyways cause it's
  source only, and in particular it makes no sense for OCI build caches.

This commit:

1. Injects the sensible mirrors into the stage object from contexts
   where it is relevant
2. Separates mirrors from cache, so that w/o mirrors download cache can
   still be used

0 files changed, 0 insertions, 0 deletions