The new step collects together all the required build artifacts and
places them in the output directory.
* Reintroduce/add the code removed from step export to preserve the
legacy export directory structure when skip_export is unset/false
* Add a place holder for a future function that will move just the VHD
files from the build directory to the output directory when
skip_export is true
* Add tests for current functionality and placeholder tests for future
functions
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