Microsoft.Build.Runtime package contains mscorlib.dll in contentFiles, which breaks sln that references it

### Steps to reproduce

There's an internal solution that I'm working on breaking down further to get a small repro, but the essence of the problem is that referencing the latest (15.8) Microsoft.Build.Runtime brings along an mscorlib.dll in contentFiles. These files end up in ResolveAssemblyReferences, and we pick up the package mscorlib, which breaks compilation.

### Expected behavior

Either the package shouldn't have an mscorlib in it, or it should end up in RAR.

### Actual behavior

It does.