Details about Problem

NuGet product used : nuget.exe/maybe dotnet

NuGet version (x.x.x.xxx): 4.3.0

Worked before? If so, with which NuGet version: 3.4.4

Detailed repro steps so we can see the same problem

1. dotnet new console

2. create a new config file and add a new source to it with name source_name

3. nuget restore -Source source_name

Expected

Restore should read the source from the config file

Actual

Restore treats the source as a relative path

Notes

More details at #5767