Parametrize `initDebugger` to take GHC session rather than use HieBios

[Saizan]

Currently we give HieBios a project root and entry file and it gives us the GHC flags and units to load. These are ultimately passed to runDebugger to setup the session (i.e. HscEnv) for the debugger.

When integrated with the persistent worker hopefully we can get a ghc session directly from it?

TODOs:

• Scope out what API the persistent worker offers, maybe mock it as code in hdb codebase?
• Define abstraction for "project session provider" that covers both HieBios and persistent worker.
• Parametrize initDebugger/debuggerThread/runDebugger to use the new abstraction.
• Implement project session provider abstraction with HieBios (default) , and mocked persistent worker API (if we have it).

[Saizan]

Skipped the mocked API part.

Fixed in b32ada0.