labsdebrepo: Fix initial run

Currently, Packages.gz is initialized as an empty file.  This makes
"apt-get update" croak until a package is added to the repository and
Packages.gz regenerated.

This change generates the initial Packages.gz by running
dpkg-scanpackages on the empty directory instead.

Change-Id: I1fd0d090cfb444e5cbd2e83259cfc3df04dddd3f
Merge "labsdebrepo: Fix initial run" into production