lib/oe/package_manager: Avoid race problems when calling list_pkgs()
list_pkgs() for rpm calls RpmPM() which would try and create a copy of the
package feed. This can be called for example from buildhistory whilst some
other task may be working on an SDK or image construction, causing the package
feed to disappear part way through an operation.
Avoid the need to copy the package index just to list the installed
packages, avoiding the race.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
