fedora: also check for the spec file in the existence check
    
    Fixes https://pagure.io/fedora-rust/rust2rpm/issue/165.
    the following workflow wasn't well supported by the check:
    
    - request compat package repo
    - fedpkg clone it
    - try to run rust2rpm for it
    - Get this message:
      Crate tiny_http0.6 is already packaged in Fedora (...)
      Re-run with --no-existence-check if you still want to convert it.