Fix/implement multi-module support for the `manifests` schema configuration

[aaron-prindle]

Currently Skaffold's manifests field does not properly implement multi-module support. See the code here: https://github.com/GoogleContainerTools/skaffold/blob/1f5af29273e2938c66311fcc22ddf0fcceeeffd7/pkg/skaffold/runner/runcontext/context.go#L295-L297

This bug tracks properly updating the manifests section of a skaffold.yaml w/ multi-module support (vs how it currently works which is just using the root modules values and disregarding the other values)

Thanks @spstarr and @gsquared94 for repro-ing and root-causing this issue

[gsquared94]

I think you have to do away with GetRenderConfig() method. And in the GroupRenderer maintain a []Hook instead of just a single Hook.
Something like this in renderer.go:

Then we need to decide if we want to iterative run each Deploy's pre and post hook around that deployer, or run all aggregate pre-hooks, then all the deployers as they are happening currently, then all the aggregate post hooks