That XML file seems to be a representation of the Sitecore.Install.PackageProject class, so I tried to generate an XSD from code using serialization on that class. However, if you use a decompile to take a look at how package building and installation works, you'll find out that Sitecore has written their own serializer for this. So I wasn't able to generate a correct XSD with the .NET serializer.
With a decompiler (I use dotPeek, freeware) you can track down a lot of information about that XML file and how it's being used by Sitecore, but I donÂ´t see a (realistic) way to extract a schema from this. If you're going to look into it, look inside Sitecore.Kernel.dll and look for the Sitecore.Install namespace.
Have you tried asking Sitecore Support? If anyone has this schema, it's them.