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.
Due to some incomplete rework of this function in an earlier commit, the
safety check for using the same directory as both the target and the
cache was inverted and was raising an error _unless_ they matched, rather
than _if_ they matched.
This change is verified by the e2etest TestInitProviders_pluginCache,
which is also updated to use the new-style cache directory layout as part
of this commit.
Martin Atkins