Support a fall-back if no match on shipped ptvsd wheels.

[ericsnowcurrently]

The plan is to use the matching shipped ptvsd wheel when debugging in the extension. However, the outcome is unclear when the extension is not able to find a matching wheel.

Here are the options I could think of to resolve the situation:

1. pip-install ptvsd (based on our requirements.txt or the user's)
2. use a source-only ptvsd (.zip or .tar.gz "sdist")
3. use the ptvsd already installed for the active interpreter

@kimadeline

[DonJayamanne]

However, the outcome is unclear when the extension is not able to find a matching wheel.

I was under the impression that we always reverted to the standard install of ptvsd (non-platform specific, as is done today).

[luabud]

We're already doing option 2 :) @karthiknadig implemented this.