package-index: index also subdirectories when using rpm

package-index: index also subdirectories when using rpm

Previously only the top-level index was created, which did not
work if PACKAGE_FEED_ARCHS whitelisting (or explicitly listing
architectures in dnf repo files by hand) was in use:
https://lists.yoctoproject.org/pipermail/yocto/2018-March/040327.html
https://bugzilla.yoctoproject.org/show_bug.cgi?id=12419

[YOCTO #12419]

(From OE-Core rev: f2a568ddb22f38114fdbc1d389c7556386ebb1fa)

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>