DAS: Remove unnecessary PluginInfo subclasses; privatize members

I don't know the history here, but there was a BuiltInPluginInfo
subclass, which has no instances (dead code), and a
DiscoveredPluginInfo class. So I deleted the former, and merged the
latter into the superclass, PluginInfo.

I also made a few members private, in PluginInfo, PluginManager, and
PluginSession. I modernized comments along the way.

This all paves the way for adding information to PluginInfo about
_new_ plugins, for use in the diagnostics pages.

Change-Id: Ib7c5deebd43e5636141ed36ddb3d5da7cf325883
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445405
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: srawlins