lib/package_manager: Improve repo artefact filtering

If you run an arm build followed by an x86 one and then ask for a
full repo to be created, it will include all of the arm and x86 packages.
testexport will then find the arm socat package rather than the x86 one
and try and run arm binaries within an x86 qemu image with no success.

The reproducer for this was:

oe-selftest -r fitimage.FitImageTests.test_initramfs_bundle runtime_test.TestImage.test_testimage_install

This patch only symlinks in the compatible package archictures rather
than all of them which fixes the failure and the resulting autobuilder
intermittent failure too.

[YOCTO #15190]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 30b45bcf)
Signed-off-by: Steve Sakoman <steve@sakoman.com>