Fixes #2587 (fixes bug introduced by #2552)

• The launch.json can contain the value $config:python.pythonPath} as the value for the python path. We need to account for this value when validating the python path before debugging.
• Also moved validation code into a separate method of the diagnostics class.

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• [no] Has a news entry file (remember to thank yourself!)
• Has unit tests & system/integration tests
• [n/a] Any new/changed dependencies in package.json are pinned (e.g. "1.2.3", not "^1.2.3" for the specified version)
• [n/a] package-lock.json has been regenerated by running npm install (if dependencies have changed)