Skip tf-compat checks for solution dependencies

Explicitly opts synthetic project references injected by solution
dependencies out of the TargetFramework-compatibility/find-best-match
dance instituted in 15.x for .NET Standard support.

Fixes the most common cause of dotnet#2661.

src/Tasks/ResolveProjectBase.cs

@@ -293,6 +293,7 @@ protected void AddSyntheticProjectReferences(string currentProjectAbsolutePath)
                     item.SetMetadata("ReferenceOutputAssembly", "false");
                     item.SetMetadata("LinkLibraryDependencies", "false");
                     item.SetMetadata("CopyLocal", "false");
+                    item.SetMetadata("SkipGetTargetFrameworkProperties", "true");
 
                     updatedProjectReferenceList.Add(item);
                 }