This removes ambiguity as to which modules are being imported, making
import slightly faster as Python doesn't need to test so many
directories for file presence.
All files should already be using absolute imports because mach command
modules aren't imported to the package they belong to: they instead
belong to the "mach" package. So relative imports shouldn't have been
used.
This extends the upstream update script with steps for pushing
local changes to upstream. The general approach is to look for all
commits to the tests directory since the last sync, rewrite those
so they apply to upstream at the last sync point, then rebase onto
the sync commit, before creating and merging a PR for each in turn.
