Root level pom.xml should exclude stub packages from the javadoc generation

otherwise they are included in `mvn javadoc:javadoc' output. e.g. see here https://jonathangiles.github.io/azure-javadocs/index.html?overview-summary.html

Start with the root pom.xml and run `mvn javadoc:javadoc` to reproduce the issue. Identify how stub packages are included in the generated output, then verify that the command excludes them, using the linked Javadoc site as a reference for the unwanted result.

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