Use new resolveDebugConfigurationWithSubstitutedVariables() API in debug provider

With the January 2020 (1.42) release, we have the option to use the new resolveDebugConfigurationWithSubstitutedVariables() API in our Docker DebugConfigurationProvider. This gets called with any variables within the configuration having been resolved, which should allow us to dispense with our own (and known incomplete) resolution logic.

In addition to that, however, with the upcoming August 2020 (1.49) release, this API will now get called after the preLaunchTask completes. (Current versions call the API before the task is invoked.) The implication of this change is that we should now have a hook that runs after the last chained task but before we have to return the transformed debugging configuration, and this means that we now have an opportunity to pass information between those chained tasks and the debugger (such as the name of the created container, maybe mapped ports, etc.), rather than have to statically declare or infer them.

This could simplify our debugging logic and something we should look into in the near future.

Start by locating the Docker DebugConfigurationProvider and its current variable-resolution logic. Review how resolveDebugConfigurationWithSubstitutedVariables() is invoked around preLaunchTask, then determine whether the custom resolution can be removed and whether information from chained tasks can reach the returned debugging configuration.