Don't `readDir`

[roberth]

The proposal suggests to use readDir to generate the attributes. This will reduce performance compared to a file that lists the directories. Note that the readDir can not be lazy, despite the sharding, because // is strict in both arguments.

The file tree flattening alone is already a worthwhile change, and the readDir solution can be applied later, when we can actually measure it.

[roberth]

We can go back or forth on this decision without much trouble, so I'll agree with whatever decision is made.