_FanOutBuild should depend on BeforeBuild and AfterBuild

[alexrp]

It's fairly surprising that making a target run before/after these extension points makes it not trigger at all for the top-level build. #68 should be addressed together with this so that such an extension target knows whether it's running in the top-level build.

[alexrp]

The current behavior actually matches what MSBuild and the .NET SDK does when you set multiple TFMs; only the inner builds trigger BeforeBuild/AfterBuild. I'm not sure why I thought otherwise.

That said, it still seems useful to provide a hook for this use case.