I tried this but some of the plugins don't work (eg. sublimelinter):
7z a '-x!.git' -tzip ../Installed\ Packages/plugin.sublime-package
I also sometimes get an invalid zipfile.
With Package Control you can run "Create Package" from the command palette.
Some packages do not work in .sublime-package format. These packages should have a .no-sublime-package file in the root of the repository so Package Control knows to extract them.