Add package exclude mechanism to specfile "parsing"

Similarly to exclude-packages in rpm-ostree manifests, we need to be able to keep Recommends on and still exclude some packages for images like Kinoite for example.

Would likely be added at https://gitlab.com/fedora/bootc/base-images-experimental/-/blob/main/build/install-imagedir?ref_type=heads#L50

See: https://docs.fedoraproject.org/en-US/quick-docs/dnf/#exclude-package