Show build backend and backend commands in verbosity mode

[abitrolly]

I would enjoy if build had some machine readable logging about how it interacts with backend, starting with backend and being used. Something like this:

$ python -m build --wheel -v
build:backend: setuptools
...

[layday]

PR welcome.

[abitrolly]

This looks like a proper place to validate backend and report its version

build/src/build/_builder.py

Lines 161 to 173 in ed9c379

	pyproject_toml_path = os.path.join(source_dir, 'pyproject.toml')
	self._build_system = _parse_build_system_table(_read_pyproject_toml(pyproject_toml_path))
	self._backend = self._build_system['build-backend']
	self._hook = pyproject_hooks.BuildBackendHookCaller(
	self._source_dir,
	self._backend,
	backend_path=self._build_system.get('backend-path'),
	python_executable=self._python_executable,
	runner=self._runner,
	)

[abitrolly]

PR welcome.

I started the implementation in #969