GitHub Enterprise private repos support

Hello there!

I'm trying to make sourcelink work with our private repos on Github. But something is not working and I can't figure out what exactly is going wrong.

I've published package `A` to our feed (we're now using MyGet). In this package I have referenced `Microsoft.SourceLink.GitHub 1.0.0` with private assets. Then I referenced `A` from another package on local machine. Visual Studio 16.6.5 set up
[like that](https://user-images.githubusercontent.com/6738342/89012687-367d9900-d31b-11ea-8f48-e8f76d55c649.png), Git Credentials Manager also configured — I can pull and push, for example. But nonetheless Visual Studio is keeps showing me "source code" from metadata and not trying to actually retrieve sources from Github.

Then I've tried to test downloaded nuget package (`A`) via sourcelink global tool. Obviously, just plain `test` didn't work, because Github is responding with `404` due to unauthorized access. Then I've tried to `test` with basic auth, providing my Github username and generated personal access token as a password. And `test` was successful! I've then changed username for stored windows credentials from "PersonalAccessToken" to my github username... But it didn't help Visual Studio to retrieve sources.

Both package `A` and code on local machine are netstandard2.0 class libraries.

Help me, please.