use correct path in rel-eng/packages if package reside in git-root

if package reside in subdirectory, then in metadata file is written
  1.0 foo/
but if the package reside in git-root, then in metadata file is written
  1.0 /
which is not relative path and if it is evaluated by some tool, it is evaluated incorrectly
This patch will change this singularity to:
  1.0 ./
use correct path in rel-eng/packages if package reside in git-root for DistributionBuilder

if package is directly in git-root, then
os.path.join('foo.git', '/') will return '/', which is not what we want'
Merge pull request #78 from xsuchy/pull-req-relative-dir

use correct path in rel-eng/packages if package reside in git-root