pkgdmg should copy the contents of each mountpoint and install from there

I’ve been seeing `EBUSY` failures when unmounting dmgs and captured the processes that still have open files with `lsof` after running puppet.

```
COMMAND    PID            USER   FD      TYPE             DEVICE  SIZE/OFF   NODE NAME
mds         43            root   15r      DIR                1,4       238      2 /private/tmp/dmg.uQT3Dp
installd   691            root    4r      REG                1,4  30079000    132 /private/tmp/dmg.uQT3Dp/Packages/MacOSX10_9_SDK.pkg
installd   691            root    5r      REG                1,4  30079000    132 /private/tmp/dmg.uQT3Dp/Packages/MacOSX10_9_SDK.pkg
installer 3164            root    4r      REG                1,4    657793    130 /private/tmp/dmg.uQT3Dp/Command Line Tools (OS X 10.9).pkg
```

`mds` is harmless but I didn’t want to unmount the dmg with `-force` and interfere with whatever `installd` or `installer` is doing with the packages.

To fix these errors I’ve copied the contents of each mountpoint, and then passed the paths of those copied pkg files to installer. This ensures the pkg files passed to installer are on the root volume and that the mounted volume can be safely ejected with `-force` thus preventing the unmount from failing because errant background processes like Spotlight have files open on the volume.

Removing the temporary directory unlinks the files but leaves the files intact so long as processes have open descriptors to them, the files are then reclaimed when the lingering descriptors are closed.