Handle preLaunch task explicitly

[anton-matosov]

Fixes #166
Fixes #129

Switch to resolveDebugConfigurationWithSubstitutedVariables in the DebugConfigurationProvider which provides automatic handling of the preLaunch task (#166) as well as variables expansion in launch config (#129).

Stop returning config from the resolvers. This fixes allows to not even start debug adapter, which we terminate right away anyways. Moreover, it fixes fairly often race condition in vscode.debugger that results in re-sending same request 5-6 times. This is most likely caused by the new debugger requests spawning from the debugger request handler.

Implement await for the extension.env in launch and attach requests for the case if reload have been triggered in the midst of the request processing. That happens in 100% of cases if preLaunch task runs catkin make/build

[ghost]

All CLA requirements met.

[anton-matosov]

While fixing #129 I have found that switching to resolveDebugConfigurationWithSubstitutedVariables handles preLaunchTask correctly and gives variables resolution. So I have revetred explicit handling of the preLaunchTask and only left changes for handling env reload and suppression of the debug adapter startup

[ooeygui]

Wow! Thank you for the contribution!
I looked through the change, and it looks good. I'll pull it down and run through some testing on Monday and pull it in.
@seanyen when you get a moment, can you review as well?

[seanyen]

Thanks for your contribution!