testers.shellcheck: refactor, update docs, and simplify tests (#385940)

## `shellcheck` {#tester-shellcheck}

Runs files through `shellcheck`, a static analysis tool for shell scripts.

Run files through `shellcheck`, a static analysis tool for shell scripts, failing if there are any issues.

:::{.example #ex-shellcheck}

# Run `testers.shellcheck`

```nix

testers.shellcheck {

  name = "shellcheck";

  name = "script";

  src = ./script.sh;

}

```

  inherit (lib) fileset;

in

testers.shellcheck {

  name = "shellcheck";

  name = "nixbsd-activate";

  src = fileset.toSource {

    root = ./.;

    fileset = fileset.unions [

### Inputs {#tester-shellcheck-inputs}

[`src` (path or string)]{#tester-shellcheck-param-src}

`name` (string, optional)

: The name of the test.

  `name` will be required at a future point because it massively improves traceability of test failures, but is kept optional for now to avoid breaking existing usages.

  Defaults to `run-shellcheck`.

  The name of the derivation produced by the tester is `shellcheck-${name}` when `name` is supplied.

`src` (path-like)

: The path to the shell script(s) to check.

  This can be a single file or a directory containing shell files.

  All files in `src` will be checked, so you may want to provide `fileset`-based source instead of a whole directory.

### Return value {#tester-shellcheck-return}

A derivation that runs `shellcheck` on the given script(s).

A derivation that runs `shellcheck` on the given script(s), producing an empty output if no issues are found.

The build will fail if `shellcheck` finds any issues.

## `shfmt` {#tester-shfmt}

  "tester-shellcheck-inputs": [

    "index.html#tester-shellcheck-inputs"

  ],

  "tester-shellcheck-param-src": [

    "index.html#tester-shellcheck-param-src"

  ],

  "tester-shellcheck-return": [

    "index.html#tester-shellcheck-return"

  ],

- The hand written `perlPackages.SearchXapian` bindings have been dropped in favor of the (mostly compatible)

  `perlPackages.Xapian`.

- [testers.shellcheck](https://nixos.org/manual/nixpkgs/unstable/#tester-shellcheck) now warns when `name` is not provided.

  The `name` argument will become mandatory in a future release.

- The `nixLog*` family of functions made available through the standard environment have been rewritten to prefix messages with both the debug level and the function name of the caller.

  The `nixLog` function, which logs unconditionally, was also re-introduced and modified to prefix messages with the function name of the caller.

  For more information, [see this PR](https://github.com/NixOS/nixpkgs/pull/370742).

  inherit (lib) fileset;

  runShellcheck = testers.shellcheck {

    name = "activation-check";

    src = fileset.toSource {

      root = ./.;

      fileset = fileset.unions [

  };

  runShellcheck = testers.shellcheck {

    name = "activation-lib";

    src = runTests.src;

  };