Fix AWS::CodeBuild::Project.FilterGroup

[jlongtine]

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-codebuild-project-filtergroup.html

I'm making some assumptions in my code about what the sub-type should look like (namely, that it ought to be a struct... in this case it is a list of lists of structs... I need to remedy this.

I have custom code that is "repairing" this currently: https://github.com/cue-sh/cfn-cue/blob/ac1d2b7f72f0bb39f66ba1197306c438ae5f822c/main.go#L76

One additional piece of complexity...

    "AWS::CodeBuild::Project.FilterGroup": {
      "Documentation": "http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-codebuild-project-filtergroup.html",
      "ItemType": "WebhookFilter",
      "Required": false,
      "Type": "List",
      "UpdateType": "Mutable"
    },

That is in a section that should all be defined as Resources, but it is defined as a Property... So it won't load correctly into the go structs...