V3 install/restore only uses credentials for index.json

[jmyersmsft]

We're still having problems with auth against v3 endpoints. Using:

• nuget.exe built from current dev branch or CI build 10477
• an authenticated v3 source containing the package 'SuperHelloLib'
• credentials stored in nuget.config,

running:

nuget install SuperHelloLib

gives the following behavior:

1. GET (index.json) (without credentials) -> 401 with auth challenge
2. GET (index.json) (with Basic credentials) -> 200
3. GET (Registration URL for SuperHelloLib) (without credentials) -> 401 with auth challenge
4. NuGet.exe gives up with the message "Failed to retrieve metadata from source: '[query URL]'.". No further HTTP requests are issued.

The same behavior is seen with restore using a packages.config

[yishaigalatzer]

This should be verified both for command line, and visual studio extension