Buildpack packaged as ZIP file

Previously, the packaged buildpack was a GZipped TAR file.
Unfortunately, the create-buildpack and update-buildpack commands only
take ZIP files.  This change updates the package task to create a ZIP
file instead of a GZipped TAR file.

[#68524510]
Merge 68524510-zip-file to master

[Completes #68524510]
Parallelize package tasks

Previously, all of the download and copying required by the package
task was done sequentially.  Especially in the case of the OFFLINE
mode downloads, this lead to sub-optimal performance.

This task makes all of the package tasks parallel, leading to a more
than 40% build-time savings.

[#68549674]
Merge 68549674-multi-task to master

[Completes #68549674]