Taglet auto-detection fails on complex dependency hierarchy

The current implementation of taglet auto-detection (initial feature request: MJAVADOC-204) lacks proper evaluation of the dependency tree of configured taglet artifacts (see <tagletArtifacts/>): each artifact on the taglet classpath is scanned separately (see AbstractJavadocMojo.addTagletsFromTagletArtifacts(..) and JavadocUtil.getTagletClassNames(..)), causing taglet class loading to fail in case its hierarchy is spread across multiple artifacts (its class loader has visibility only on a single artifact at a time, in the wrong assumption that no intermediate artifact lies between the Taglet interface and its implementations).