Add a top-level node for analyzers (#9906)

This is the first step, and just estabilishes a skeleton. The end goal
is organize our analyzers under this node because:
1. We're writing some 'top level' analyzers that depend on everything
else
2. We want to be able to service analyzers on the SDK schedule
(independent of runtime).

Next, we'll merge the MVC analyzers into this assembly since there's no
reason for them to be separate. The MVC API analyzers will remain a
separate package, but under this node. The component analyzers will
remain separate as they need to ship as a package, but will move under
this node as well.