Plugins don't work with -fexternal-interpreter.

The current plan to fix this is:

• Support loading two different UnitState (available units): one for the target, one for plugins
• Add command-line flags (-plugin-package-db, etc.) to build the plugins UnitState
• Refactor many functions to explicitly pass Platform configuration (Platform, ways, etc.) as arguments. Currently we often pass DynFlags and callee functions implicitly use the UnitState of the target platform. It doesn't compose well and we want to be explicit about the platform we are using (target or host) (see also #17957).
• Pretty-printing of Unit (via its Outputable instance) implicitly queries the UnitState of the target platform (via sdocWithDynFlags). We need to remove this Outputable instance.