tests.nixpkgs-check-by-name: Improve check clarity

### File structure checks

- `pkgs/by-name` must only contain subdirectories of the form `${shard}/${name}`, called _package directories_.

- The `name`'s of package directories must be unique when lowercased

- The `name`'s of package directories must be unique when lowercased.

- `name` is a string only consisting of the ASCII characters `a-z`, `A-Z`, `0-9`, `-` or `_`.

- `shard` is the lowercased first two letters of `name`, expressed in Nix: `shard = toLower (substring 0 2 name)`.

- Each package directory must contain a `package.nix` file and may contain arbitrary other files.

- Each package directory must not refer to files outside itself using symlinks or Nix path expressions.

### Nix evaluation checks

- `pkgs.${name}` is defined as `callPackage pkgs/by-name/${shard}/${name}/package.nix args` for some `args`.

- `pkgs.lib.isDerivation pkgs.${name}` is `true`.

- For each package directory, the `pkgs.${name}` attribute must be defined as `callPackage pkgs/by-name/${shard}/${name}/package.nix args` for some `args`.

- For each package directory, `pkgs.lib.isDerivation pkgs.${name}` must be `true`.

### Ratchet checks

The current ratchets are:

- If `pkgs.${name}` is not auto-called from `pkgs/by-name`, the `args` in its `callPackage` must not be empty,

- New manual definitions of `pkgs.${name}` (e.g. in `pkgs/top-level/all-packages.nix`) with `args = { }`

  (see [nix evaluation checks](#nix-evaluation-checks)) must not be introduced.

## Development