Support package.config

[bouassaba]

Reference: https://docs.microsoft.com/en-us/nuget/reference/nuget-config-file

[sikri-eic]

I think NuGet package sources are stored in nuget.config file (https://docs.microsoft.com/en-us/nuget/reference/nuget-config-file). And one can use the packageSourceCredentials to get crendentials per source.