Auto merge of #2828 - bennofs:cargo-repackage, r=alexcrichton

cargo package: overwrite existing tarballs

Previously, cargo package did not do anything if a tarball already
existed. This is wrong, because the source may have changed and cargo
does not do any dependency tracking for package tarballs yet, so it did
not notice this.

This commit changes cargo package to always overwrite existing tarballs,
which works fine until proper dependency tracking is implemented.

Fixes #2799