Custom Maven extensions are not fully supported

When the dependency type or the project packaging is custom, Extension Manager does not resolve properly the actual extension file (and the extension type).

Since XCOMMONS-3003 (16.4.0), the workaround is to install the JAR which contains a plexus components.xml definition of the extensions in the root namespace and to restart (it will be loaded as it of was one of the standard Maven extensions).

Note: in Maven extensions can be defined both explicitly in <extensions> but also through though plugin with the <extension> flag enabled