[FLOC-1020] Remove DelayedVersion and associated logic

This branch limits `admin/build-package` to building only a supplied source directory.

This allows me to remove the GetPackageVersion step and the associated DelayedVersion, which existed only so that the version could be got from a package installed via URL and passed to later build steps.

Fixes https://clusterhq.atlassian.net/browse/FLOC-1020
Running a command here feels like cheating.