Ib3f22e70b7 upgraded to new Buck version but broke sources download
during Eclipse project creation. That's because the namespacing of
genrule() artifact. To rectify and to avoid querying the actual
location of source artifact from Buck for every source library with:
$ buck targets --show_output //lib:foo__download_src | awk '{print $2}'
switch from using genrule() to prebuilt_jar() to induce the source from
binary artifact, because the location is stable for this rule.
Change-Id: I153d3e31b4f7098ddd5157b7b1bba17529c83b32
805ef02aa3