Improve plugin discovery

[omry]

Plugin discovery is discovering the same plugins multiple times, this cases them to be imported multiple times which results in slow startup (especially if the plugins are slow to import like some of the new plugins (Ax sweeper)).

The plugin discovery should only happen once, and cached results should be used subsequently.

This will involve converting the static hydra.core.plugins.Plugins class to a Singleton, which is an incompatible API change.

[omry]

done in #490