Add 'preserveExisting()' as addition to 'patchRealModule()'

[jjohannes]

To make it easier to add an entry to an existing module-info.class, that is in general working well, without having to repeat the existing content. A good example for this are requires static entries that may have been missed as they do not influence the runtime behavior of the Module. For example:

  module("org.apache.logging.log4j:log4j-api", "org.apache.logging.log4j") {
        patchRealModule()
        preserveExisting()
        requiresStatic("com.google.errorprone.annotations")
        requiresStatic("biz.aQute.bnd.annotation")
    }