This is a breaking change. Previously, packages named `packageset.foo`
would be exposed in the fake derivation channel as `packageset-foo`.
Presumably this was done to avoid needing to track attribute sets, and
to avoid the complexity. I think this now correctly handles the
complexity and properly mirrors the input expressions layout.
By moving the tests subdirectory to t, we gain the ability to run `yath
test` with no arguments from inside `nix develop` in the root of the
the repo.
(`nix develop` is necessary in order to set the proper env vars for
`yath` to find our test libraries.)
