Separate move path tracking between borrowck and drop elaboration. The primary goal of this PR is to skip creating a `MovePathIndex` for path that do not need dropping in drop elaboration. The 2 first commits are cleanups. The next 2 commits displace `move` errors from move-path builder to borrowck. Move-path builder keeps the same logic, but does not carry error information any more. The remaining commits allow to filter `MovePathIndex` creation according to types. This is used in drop elaboration, to avoid computing dataflow for paths that do not need dropping.