Create meta assembly for Microsoft.Build.Engine
Update projects
Move SolutionParser to Mono.XBuild.Engine and make it public
Move AssemblyLoadInfo to Mono.XBuild.Engine
Add some deprecation warnings
Remove TaskItem from Microsoft.Build.dll build

It's not used anywhere. If someone needs it in the future they can pick up
its implementor Mono.XBuild.Utilities.TaskItemImpl from the
Mono.XBuild.Utilities assembly.
Rectify Microsoft.Build csproj file.
Use MSBuildUtils rather than Ms.Build.Engine.Utilities