This simplifies the way that we publish files to our network drop share.

Changes:
* Instead of explicitly listing every file that needs to publish, use directories to classify packages and artifacts into different categories.
* Add documentation for the expected layout of artifacts/
* Remove the need for static analysis to determine which packages go to which project
* Add the MSBuild property "IsProductPackage" to .csproj files which ship as a package to NuGet.org.