Blender Git Loki

Link/Append: Move main linking code (from WM) to use new instantiation code in BKE.

This removes the last main usage of BLO's instantiation code.

Also required some limited refactoring of BKE_blendfile_link_append's
instantiation to make it more modular, and usable by both linking and
appending code paths.

NOTE: This should be a non-behavioral change commit.

Part of T91414: Unify link/append between WM operators and BPY context
manager API, and cleanup usages of `BKE_library_make_local`.