Automatically infer local and remote path mappings for remote debugging

[DonJayamanne]

Currently we expect users to provide the local and remote path mappings when setting up remote debugging.
Then there's source references.

If we try to automatically infer local and remote path mappings, then source references might not work.
This issue is merely a reminder to look into the ability to automatically infer the path mappings.

microsoft/vscode#47914

[DonJayamanne]

We'll need to come up with a solution for this before we release the new debugger, as the default values in launch.json will affect this.

[brettcannon]

If we can then great, otherwise "oh well" 😉

[DonJayamanne]

Upstream issue microsoft/ptvsd#691