buildifier: decouple runner scripts from binaries (#1398)

This change aims at making `buildtools/buildifier` more modular and
easier to consume by enabling use of prebuilt binaries without pulling
in unnecessary dependencies - while strengthening upstream as the
authoritative home for runner script templates.

Today, `buildifier/BUILD.bazel` exposes two different sets of rules:
- exports for runner script templates (`runner.bash.template`,
  `runner.bat.template`) as well as `README.md`,
- `go` build & `sh` test rules which require external repos (`rules_go`,
  `rules_shell`) even for consumers who don't need to build the
  `buildifier` binary.

It is worth noticing there are two distinct community efforts on runner
scripts, the present/upstream `bazelbuild/buildtools` and the derived
`keith/buildifier-prebuilt` (Bazel Central Registry module).
These drift apart over time, leading to unsynchronized contents
(`factory.bzl`, `runner.bash.template`) and efforts, e.g. on Windows
support:
- https://github.com/bazelbuild/buildtools/pull/1230
- https://github.com/keith/buildifier-prebuilt/pull/89 (open for a year)

Intended benefits of this change:
- decoupling: by moving `go_binary` and `sh_test` targets into
  `buildifier/cmd`, the top-level `buildifier/BUILD.bazel` no longer
  requires `rules_go` nor `rules_shell`,
- flexibility: users can either build binaries from source (via
  `//buildifier/cmd`) or configure rules to use prebuilt binaries (e.g.
  via `multitool`) while still consuming the same upstream runner script
  templates,
- backward compatibility: aliases keep `//buildifier:buildifier*`
  working by delegating to `//buildifier/cmd:buildifier*`.