In the new design the ProviderSource is decided by package main, not by
the "command" package, and so making sure the vendor directory is included
is the responsibility of that package instead. Therefore we can no longer
test this at the "command" package level, but we'll retain a test for it
in e2etests to record that it isn't currently working, so that we have
a prompt to fix it before releasing.
Martin Atkins