[FIX] packaging: verify repodata exists

The rpm packaging fails to generate a repository when the repodata
directory does not exists previously to the build.

The reason is that the package.py script removes the repodata before
rpm repo generation without verifying the the directory exists.

That kind of corner case happens when a new odoo version comes out.
With this commit, the script now verifies that the directory exists.

Also, while at it, a time stamp based on seconds is added to the default
build directory name to allow to build more than one time the same day
without removing the build directory. It's mainly a testing use case,
when the packaging script is tested, the build dir is usually kept for
debugging purposes.

closes odoo/odoo#78386

Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>