re-organize sources to follow BATS plugins best practice

[e-picas]

To follow official BATS libs best practice and installation process, I propose the followings:

• put the Bash sources in a src/ directory
• add a load.bash entrypoint file for BATS classic loading

I've tested it in a docker image with bats and other libraries and it works fine.

---

The BATS default method to install a library is something like (cf. https://github.com/bats-core/bats-core/blob/master/docker/install_libs.sh):

  wget -q -O "${_tmpdir}/${_tarball}" ${BASE_BATS_URL}/bats-"${_libname}"/archive/refs/tags/v"${_libvers}".tar.gz

  tar -xz -f "${_tmpdir}/${_tarball}" -C "${_tmpdir}/"

  install -Dm755 "${_tmpdir}/${_libdir}/load.bash" "${_target}/bats-${_libname}/load.bash"
  for fn in "${_tmpdir}/${_libdir}"/src/*.bash
  do
      install -Dm755 "$fn" "${_target}/bats-${_libname}/src/$(basename "$fn")"
  done