lib: query sourcepkg by basename
    
    Otherwise we end up with returning random packages with same prefix.
    $ curl --silent http://127.0.0.1:8080/rawhide/srcpkg/python
    ...
      "basename": "python-oslo-config",
    ...
    
    Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>