Support ExcludeFromSourceBuild when using PackAsTool

In dotnet/sdk/src/Tests/Directory.Build.targets, PackAsTool is being set to true. This causes the Pack to use PackTool, instead of Pack.

In RepoToolset, if ExcludeFromSourceBuild is set to true, which it is for these tests, instead of importing the real Pack target, it imports an empty Pack target which basically noops for the project. But, since the SDK test projects switched to using PackTool, it isn’t noop’d. The easy solution is to add PackTool to the list of empty imports, but the other thing that happens when setting PackAsTool to true is that TargetsForTfmSpecificContentInPackage gets PackTool included, which then gets set as a DependsOnTarget for some other targets that get included and then fail because they expect certain artifacts to be present.