BuildSRPMFromSCMTask: Support auto-selecting a matching specfile name
    
    There are cases where having two files with the .spec suffix in the
    same dist-git repository makes sense, such as maintaining a common
    dist-git structure for different distributions where the package
    name might be different.
    
    This patch would allow the build-system to use the matching spec
    file name for the current SCM checkout path if it exists.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>