"Fold" rust-srpm-macros into rust-packaging
    
    It's quite annoying (and against the guidelines) to have to packages
    built from the same source tarball. Let's simplify our life by
    building rust-srpm-macros here too.
    
    Currently %{rust_arches} includes all architectures supported in main
    koji, so '%ifarch %{rust_arches}' is effectively a noop. But rust.spec
    is conditionalized with ExclusiveArch:%{rust_arches}, so I kept this
    conditionalization here too, and the python packages are still "archful".
    
    For testing, I replaced '%ifarch %{rust_arches}' with '%ifarch x86_64
    ppc64l' and did a scratch build in koji. As expected, rust-srpm-macros
    is available everywhere, and the other packages only on the specified
    architectures.
    
    If this is merged, the separate rust-srpm-macros package should be
    retired. Since an identically named rpm is provided, no action apart
    from a rebuild is necessary.