The AnalyserMetadataHandler is executed too often

Consider the following scenario:

1. In feature A, the analyser-metadata extension is configured with:

{
    "analyser-metadata:JSON|true":
    {
      "org.apache.sling:org.apache.sling.api:2.0.6" : {
        "manifest": null,
        "report": {
          "error": false,
          "warning": false
        }
      }
}

2. During aggregation, feature A is processed with the analyser-metadata extension, resulting in feature B.

This will result in a the "report" key being retained but not the manifest.

3. At a later point, feature B is aggregated again, resulting in feature C.

During the aggregation process, the manifest will be cached because the "manifest": null key was not preserved during the first aggregation. This is unexpected and should be fixed.

The metadata handler will no longer be registered for being picked up by the ServiceLoader and must instead be configured manually.