These changes implement `GetRequiredPlugins` for Go using a registry
mechanism and an alternate entry point for `pulumi.Run`. Packages that
require plugins are expected to register themselves with the Pulumi SDK.
When `pulumi.Run` is used and the `PULUMI_PLUGINS` envvar is truthy, the
program will dump a JSON-encoded description of its required plugins to
stdout. The language host then uses this description to respond to