Update `RepackageMojo` and supporting classes so that `exclusions`
on the repackage goal apply to both the contributed libraries and any
existing jar entries already contained in the original war.

Prior to this commit, exclusions would apply to contributed jars (for
example, those in `WEB-INF/lib-provided`) but not jars that were
packaged directly into `WEB-INF/lib` by the war plugin

Fixes gh-15808
Co-authored-by: Phillip Webb <pwebb@vmware.com>