[Yum] Patch to fix yumdownloader bug 469 (source of subpackages)

[Kyle Galloway]

This patch fixes the problem
(https://devel.linux.duke.edu/bugzilla/show_bug.cgi?id=469) where
Yumdownloader wouldn't find anything when the requested source is inside
a superpackage.  For instance, here the program now downloads glibc when
asked for the source of one of its subpackages, nscd.  There are useful
comments in the code to decipher how it works.  Below is the terminal
output when the case in the bug report is run with the updated code.

[kgallowa at ________ yum-utils]$ python yumdownloader.py --source nscd
jpackage-generic                                                     [1/8]
livna                                                                [2/8]
flash                                                                [3/8]
jpackage-fedora                                                      [4/8]
updates                                                              [5/8]
core                                                                 [6/8]
core-source                                                          [7/8]
extras                                                               [8/8]
glibc-2.4-4.src.rpm       100% |=========================|  15 MB    08:52

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