Modify CF template before nested stacks get created

[rmbl]

We're using serverless-plugin-split-stacks to split the resulting CF template into nested stacks. This occurs on after:aws:package:finalize:mergeCustomProviderResources which is part of package:finalize. Because of this the Stage gets added too late and leads to errors when deploying it to AWS.

This PR moves the modifications from this plugin to the beginning of package:finalize to fix that.

[juanjoDiaz]

Is there anything blocking this from being merged and released?

[rmbl]

I should probably fix the tests. Other than that it has been working since then over a few hundred deployments for us.