Adding a dependency to an external (dotnet) repo

I recently got an error message when building the winforms repo that looked like this:

C:\Program Files (x86)\Microsoft Visual Studio\2019\IntPreview\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(3056,5): error : MSB3822: Non-string resources require the System.Resources.Extensions assembly at runtime, but it was not found in this project's references. [E:\src\repos\github\winforms\src\System.Windows.Forms\src\System.Windows.Forms.csproj]

Here are the steps I did to add this dependency to our repo:

1. I added a package reference to System.Resources.Extensions with the version set to a variable that was not yet defined.

<PackageReference Include="System.Resources.Extensions" Version="$(SystemResourcesExtensionsPackageVersion)" />

2. Now I had to find where this package comes from.
   • Version.Details.xml contains all the dependencies we consume from other repos, so I started there.
   • I saw a bunch of System.* packages that come from corefx so I tried looking there
   • Sure enough, I found https://github.com/dotnet/corefx/tree/master/src/System.Resources.Extensions