Merge pull request #278687 from keanuk/check-by-name-multiple-failures

- `default.nix`:

  Always contains

  ```nix

  import ../mock-nixpkgs.nix { root = ./.; }

  import <test-nixpkgs> { root = ./.; }

  ```

  which makes

  ```

    let root = Root::parse(&contents);

    if let Some(error) = root.errors().first() {

        // NOTE: There's now another Nixpkgs CI check to make sure all changed Nix files parse

        // correctly, though that uses mainline Nix instead of rnix, so it doesn't give the same

        // errors. In the future we should unify these two checks, ideally moving the other CI

        // check into this tool as well and checking for both mainline Nix and rnix.

        return Ok(NixpkgsProblem::CouldNotParseNix {

            relative_package_dir: relative_package_dir.to_path_buf(),

            subpath: subpath.to_path_buf(),

pkgs/by-name/A: Invalid directory name "A", must be at most 2 ASCII characters consisting of a-z, 0-9, "-" or "_".

pkgs/by-name/A/fo@: Invalid package directory name "fo@", must be ASCII characters consisting of a-z, A-Z, 0-9, "-" or "_".

pkgs/by-name/A/fo@: Path foo is a symlink which cannot be resolved: No such file or directory (os error 2).

pkgs/by-name/A/fo@: Path package.nix is a symlink pointing to a path outside the directory of that package.

pkgs/by-name/aa: This is a file, but it should be a directory.

pkgs/by-name/ba/bar: This path is a file, but it should be a directory.

pkgs/by-name/ba/baz: "package.nix" must be a file.

pkgs/by-name/ba/foo: Incorrect directory location, should be pkgs/by-name/fo/foo instead.

pkgs/by-name/ba/foo: File package.nix at line 2 contains the path expression "/bar" which cannot be resolved: No such file or directory (os error 2).

pkgs/by-name/ba/foo: File package.nix at line 3 contains the path expression "../." which may point outside the directory of that package.

pkgs/by-name/ba/foo: File package.nix at line 4 contains the nix search path expression "<nixpkgs>" which may point outside the directory of that package.

pkgs/by-name/ba/foo: File package.nix at line 5 contains the path expression "./${"test"}", which is not yet supported and may point outside the directory of that package.

pkgs/by-name/fo/foo: Missing required "package.nix" file.

none

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