Fix requires and directory/module layout

Here is the third round. I still didn’t have time to write a rationale for all the changes, so I’m sending just pull request and will comment it later (hopefully in 1–2 days).

In the meanwhile, please read the updated usage instructions in readme. tl;dr: use `./bin/asciidoctor-latex foo.adoc`.

Please note that GitHub doesn’t follow renamed files in diff, so it’s better to review it in a classic way using `git diff`.