Add the ability to automatically buildrequire default modules defined by the buildrequired base module
    
    A base module can set xmd.mbs.default_modules_url, which contains a
    URL to a list of modules in the format of name:stream separated by
    new lines. When a module buildrequires this base module, the list
    of default modules are added as buildrequires of the module automatically
    unless there are conflicting streams or the default module is not
    in the MBS database.