VaultPackageAssembler should not implement EntryHandler

kpauls, as stated with the PR for SLING-10754 i find it quite confusing the VaultPackageAssembler implements EntryHandler and is used as default in ContentPackage2FeatureModelConverter. even more so as the assembler is accessed by other EntryHandler implementations to add package entries that don't end up in repo-init.
while the effect is likely the same if the EntryHandler methods get moved to a separate DefaultHandler, it would IMHO make the code easier to read.