clone: Create metadata for empty components
In certain scenarios, mirror components (e.g. trusty-security)
are present in both packetary configuration input, and in target
system's apt source definitions, but contain no packages of interest.
For such repository/requirements combinations, packetary currently
skips creating metadata (Release, Packages) for those components,
which leads to the partial mirror missing some critical files for apt,
(an empty file would be enough).
e.g.: Using packetary to create a partial Ubuntu mirror, then trying
to build a bootstrap image from the new mirror leads to:
W: Failed to fetch http://127.0.0.1:8080/mirrors/ubuntu/dists/\
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>