Flatten image layers

Summary:
Plugins are installed according the build log,
but files are missing in the final image.

This layer reorganization fixes the issue.

This is a follow-up for rDETHER6a0a2693fdc3.

Ref T1419.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1419

Differential Revision: http://devcentral.nasqueron.org/D1735