[pull-debian-source] wildcard support

It would be nice if pull-debian-source would support a wildcard. In any case, it shouldn't crash.

$ pull-debian-source flufl*
Traceback (most recent call last):
  File "/usr/bin/pull-debian-source", line 134, in <module>
    main()
  File "/usr/bin/pull-debian-source", line 108, in main
    source_package = source_package_for(package, suite)
  File "/usr/bin/pull-debian-source", line 59, in source_package_for
    data = json.load(urllib2.urlopen(url))['r']
  File "/usr/lib/python2.7/json/__init__.py", line 278, in load
    **kw)
  File "/usr/lib/python2.7/json/__init__.py", line 326, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 382, in raw_decode
    obj, end = self.scan_once(s, idx)
ValueError: Expecting object: line 1 column 5 (char 5)