Fix #667625 #667614 and #667636
    
    - Install maven metadata so they are not downloaded when mvn is run (667625)
    
      repo-metadata.tar.xz will have to be updated as additional plugins need new metadata
    
    - Make sure JavadirWorkspaceReader is used when building model (patch update)
    
      Needed because WorkspaceReader is not used early when building mvn project
      and this causes problems
    
    - JavadirWorkspaceReader doesn't pretend to resolve non-existent artifacts anymore
    
      Instead it returns null (artifact not found) so that reactor/local resolver can work as expected