Previously, we would prefer a plugin on the $PATH which is more or
less always the case for people hacking on `pulumi`. Later, when we
went to check the loaded plugin version matched the one we requested,
we fail.
Now, if we have a version, we'll first consult the local plugin
cache. If that fails, we'll fall back to the $PATH as we used to.
When we are loading a plugin without a version, we continue to use the
one on the $PATH (without testing the cache) on the assumption it is
newer.
In addition, we've turned the "plugin versions are mis-matched" from
an error into a warning. We expect that we'll only ever see this
warning when something strange is going on (since in the normal case,
we'll have found the exact version in the cache) but having it not
hard fail does help in development cases.
Fixes#977
ed7a4d9157