Refactor direct dependence on `latexmk` module

Extract the current direct usage of `latexmk` in latex::build(), behind a new
latex::getBuilder() method. This is the first step towards introducing new
LaTeX builders.
Improve specs by using latex::getBuilder method

By swapping out the real `latexmk` builder with a fake builder, we've
drastically improved spec timings. The actual `latexmk` builder spec still uses
the real implementation to ensure everything works end-to-end.
Remove unnecessary whitespace :lipstick: