Warn when nuget.org v2 is used instead of the v3 endpoint

[emgarten]

The client should log a warning during restore if https://www.nuget.org/api/v2/ is used and suggest that https://api.nuget.org/v3/index.json should be used instead.

Benefits of v3:

• Trusted source support for package signing
• Improved performance on the client and reduced server load when using flatcontainer (static files) instead of the odata feed.

If a restore has both the v2 and v3 endpoints for nuget.org restore could also warn or error since this is redundant and only slows down restore.