Remove platform hax

We enabled some hax to get a self-hosted atbuild on 1.1.  However, since 1.2 is released, these hacks are no longer required.

* Use version field from build.atpkg
* Use only-platforms to segregate linux/osx packaging behavior
* Use `package` noptask as an alias for the platform packager

Resolve #96
Status Job ID Name Coverage
  Build
passed #3570
autoscale-linux
linux

00:02:41

passed #3571
openswift
osx

00:00:42

passed #3566
autoscale-linux
linux

00:04:47

passed #3567
openswift
osx

00:00:42