Infer AssemblyFile if TaskFactory is used

It's no small annoyance to have to know the full file name as well as versioning scheme of the tasks assembly, which has been a moving target in almost every version of MSBuild.

To the point that I ended up creating a nuget package that gives me a $(CodeTaskAssembly) that works reliablely across all MSBuild versions.

So, how about just having MSBuild automatically fill the AssemblyFile for me whenever I use a TaskFactory="CodeTaskFactory" attribute in a UsingTask? Or maybe even more concise yet, just give me a separate element name for it, like UsingCodeTask or something and do away with the attributes altogether :)

May be a superset of what's asked for in #897?