Add support for remote source references #1322

Scenarios:

1. Debugging std libs or similar
2. Attaching to a local process
3. Debug a remote application without any source on the client side (all is one of, breakpoint(), ptvsd.wait_for_attach or ptvsd.break_into_debugger or an exception).

Upstream issue microsoft/ptvsd#318

I'd consider item 2 to be a great feature for release (I'm assuming this isn't already possible in PTVS, I know it isn't in VSC Python).

• We shouldn't have to make any change to VSC (except for making the remoteRoot setting optional in launch.json)