Retain order of overridden update sites

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
WIP: Add an API to remove update sites properly

When removing an update site, we really *need* to unshadow shadowed
files... otherwise, surprising things start to happen, such as uploading
incompatible lists of superseded previous versions.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
wip
wip: test

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>