undo-files: add a undoname closure to the _write_undo method We will also needs it when the transaction will take care of the other journal files, which is soon™.

undo-files: cleanup backup when cleaning undos Previously, the backups were left behind, by operation cleaning the undo's like strip, narrow and stream clone. The remaining elevant in the room is the transaction itself, who does not properly cleanup undo backup before copying the new ones.

undo-files: factor the vfs map in a repository property We define it in multiple locations and inconsistencies are appearing. So we now have a single definition point.

undo-files: add a utility function to read the backup-files definition We will need it in multiple places. so lets factor the logic around.

undo-files: use the cleanup function in streamclone Lets use the same code, so that we can fix things only once.

undo-files: also remove the undo.backupfiles The undo.backupfiles is dealt is directly managed by the transaction instead of going through the `localrepo.undofiles`. We start doing minimal management for it before using `cleanup_undo_files` on more situation. Proper handling of it is an intermediate goal of this series.

undo-files: use the cleanup function in narrow Lets use the same code, so that we can fix things only once.

undo-files: extract the cleanup code from strip in a function This logic is duplicated in multiple places and it missing some important parts. So lets start dealing with the duplication first.

run-tests: fix a crash when using the coverage options 35bf7f23b84c attempted to transition away from `distutils`, but the `packaging` code lacks `StrictVersion`. I have no idea when `packaging.version` became available, but I have it in python 3.6, so that should be good enough. For some reason, the import checker thinks this is a local import, and needs help to decide otherwise. Alternately we could ditch the version check entirely, because `coverage` is currently at 7.2.1, and the original check was added back in 2010.

rust: fix building on macOS (issue6801) The VFS change is copied over from Cargo, and likely to apply to other platforms as well. The dirstate change is essentially a replay of 440972d2175d, which was reverted in e98fd81bb151, part of !383, to silence some clippy warnings.