Improve default interpreter path suggestion in quickpick UI

[karthiknadig]

• Move "star" icon next to the suggested interpreter in quickpick list instead of showing it as a new entry
• Show default Interpreter path suggestion only if user has set python.defaultInterpreterPath entry
• In the above case show default Interpreter path entry at the top of the list instead, but below "Enter interpreter path" suggestion. Make it the active entry.

https://github.com/microsoft/vscode-python-internalbacklog/issues/277