ubuntu: Ignore packages in packagelist.in which are missing.

For distros using 'yum', packages which cannot be found are just
ignored.  This allows us to include for example 'e4fsprogs' on the
list even though that only exists on RHEL 5.

However for Debian-derived distros, missing packages would cause
apt to fail.  This was particularly a problem when building Ubuntu
since it doesn't have as complete a list of packages, and the list
changed a lot between releases.

This commit adds a check first for missing packages, ignoring those
and thus acting more like 'yum'.