This change lets plugin versions to float in two ways:
1) If a `pulumi plugin install` detects a newer version is available
already, there's no need to download and install the older version.
2) If the engine attempts to load a plugin at a particular version,
if a newer version is available, it will be accepted without error.
As part of this, we permit $PATH to have the final say when determining
which version to accept. That is, it can always override the choice.
Note that I highly suspect, in the limit, that we'll want to stop doing
this for major version incompatibilities. For now, since we don't
envision any such version changes imminently, this will suffice.