Rename _pkg_extra_* macros to _distro_extra_* macros
    
    The two intended uses cases for the _pkg_extra_* macros were to
    make it easier for packagers to add new compile flags for use
    with their package and also to make it easier to do distro wide
    experiments with new flags.
    
    However, it was pointed out on the mailing list[1] that you
    can't satisfy both of these uses cases at the same time with
    just one set of macros.  For example, if a packager uses
    _pkg_extra_* macros to add flags specific to their own
    package, then someone using _pkg_extra_* macros to
    apply a new flag distro wide would override the package
    specific flag.
    
    I feel like the distro-wide use case is much more important,
    so rather than create two sets of new flags, one for each use
    case, I think it's best to rename the _pkg_extra_* macros
    to _distro_extra_* and document that they are only meant
    to be used for adding distro-wide flags and packagers should
    not use this macro.
    
    [1] https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/TOG5RHWPS3VYMM52HFGZOUJVRCGZ7VXB/