Split artifacts installed on system in random order

Currently ybd stages things in the build directory in the correct deterministic order.

When assembling a "system" however, it will:

• First stage all dependencies into a system staging area
• Run system integration commands (let's ignore that these will create files not collected in the next step...)
• Loop over the stratum in the system and move only the artifacts, directly from the staged system into a /.inst/ directory

This last step is done in random order, which can cause problems:

• When there are overlaps, we dont know what ends up in the final system
• When there are symlink directories, it's not entirely clear to me if this is fail safe

Interestingly, these artifacts are moved into the system install directory in a random order, not only is it not in staging order, but I've just run ybd twice in a row and noticed it did not choose the same order twice in a row.