Sharding

[infinisil]

We need to decide whether we want to "shard" or "prefix" the package directories. This means that hello would live in e.g. pkgs/auto/he/hello instead of just pkgs/auto/hello

Advantages of this approach:

• We can avoid hitting GitHub's limit of 1000 listable files
• git performance might be better

Disadvantages:

• Nix evaluation might be slower since a lot of directories need to be read
• It becomes more complicated, having to handle edge cases and coming up with a sharding scheme that works well

[infinisil]

Actually we can just reuse issue #1 for this