Checkout the version-tag

Uses git reset --hard tags/"version-tag"
Decide to build based on :stability property
Respect version number from package's source files

1. If any of the source files contains a version number, it is found and
returned instead of the timestamp.
2. This string is prepended with "Z" so that it will sort in front of
timestamps.
3. The prepended "Z" is then removed.