set sourcelink to be a private asset so that it doesn't get added to package dependencies

[baronfel]

Fixes #1376 to remove the errant sourcelink dependency.

After this change, the nuspec contains the following dependency groups:

<dependencies>
  <group targetFramework=".NETStandard2.0">
    <dependency id="FSharp.Core" version="4.7.2" exclude="Build,Analyzers" />
  </group>
</dependencies>

[baronfel]

Test errors are from the worldbank data, but don't represent a problem with the package.

[cartermp]

Thanks @baronfel