E.g. running `nix run` on the empty flake

  {
    outputs = { self }: { };
  }

now prints

  error: flake 'path:/home/eelco/Dev/nix/flake' does not provide attribute 'apps.x86_64-linux.default' or 'packages.x86_64-linux.default'

OTOH, if there are no schemas, as in

  {
    outputs = { self }: { schemas = { }; };
  }

you get

  error: Flake 'path:/home/eelco/Dev/nix/flake' does not have any schema that provides a default output for the role(s) nix-run.

`derivation` was fishy because it allows the schema to transform the
derivation, or to construct an entirely new derivation. That might be
useful sometimes, but flake schemas should describe what's in the
output, not add entirely new stuff.

We don't show derivation names/versions by default because computing
the name attribute is potentially very expensive (e.g. for a NixOS
configuration, it can take several seconds).

Flake schemas

(cherry picked from commit ac2dd58)

Merge builtins.wasm and builtins.wasi into one function

…ransfer

libstore/filetransfer: Fix double callback on enqueueFileTransfer that is shutting down