Failed to restore NuGets from local source with trailing backslash

Details about Problem

NuGet product used: NuGet.Configuration assemblies

NuGet version (4.8.0):

OS version: macOS 10.13.6

Worked before? If so, with which NuGet version: N/A

Detailed repro steps so we can see the same problem

Originally filed on Developer Community against Visual Studio for Mac.

1. Have a NuGet.Config file that uses a backslash for a local package source.
2. With a custom NuGet package in the package source that is defined with a backslash and a project that uses it.
3. Get the package source information.
4. Package source information contains a backslash.

Sample test project - https://developercommunity.visualstudio.com/content/problem/374264/failed-to-restore-nugets-from-local-source-with-tr.html

Not sure if this is would be considered a bug in NuGet. NuGet could fix up Windows path separators so things work on the Mac.

Sample Project

https://github.com/mrward/TestNuGetPackageSourceBackslashMac