Add directories to current cache context

[davidfowl]

• Flow the ICacheContextAccessor through the DI system
• Watch all directories that source files are currently in (including)
  the project directory.
  When new files are added, diagnostics data from design time host is not getting refreshed properly #552

[Eilon]

, but also a question: I'm certainly not familiar with the depths of the design of this, but can/should the caching stuff be plumbed as a service (I see an IServiceProvider passed around). By default, no caching. But for DTH you get the appropriate implementation. Thoughts? Crazy?

[davidfowl]

@Eilon almost did that but I think we need some caching in the runtime by default. Things like razor still rely on runtime compilation so keeping the cached compilations around is nice for that. We don;t need fully blown cache at runtime though, just something super basic (like a dictionary).