refactor(install): make the installer independent of the current working directory
The installer resolved the autoloader, the config check and the step files
relative to the current working directory, and the translation domain relative
to ROOT_PATH. They now come from __DIR__ or from the path constants.
step_3 read the .env file through dirname(__DIR__, 2), a hardcoded depth that
would silently break and drop the database autofill under Docker. step_2 lists
the writable directories as absolute paths and labels them by subtracting
ROOT_PATH; PUBLIC_PATH joins the list, the sitemap and robots.txt are written
there.