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.
f2ae3a7afc