JPAFacet.isInstalled() should only check for descriptor's existence

The JPAFacet implementations depend on certain libraries to be installed to be enabled.

However, if a project already has EclipseLink JARs or even OpenJPA JARs, the JPA Facet is not recognized as installed. Therefore, it would be much better if the JPAFacet logic for installation (isInstalled()) would just check for persistence descriptor existence and the version attribute.