Add an option to ignore dev dependencies

[clue]

Currently, we analyze every single package and add all of them to a single graph, no matter if it's a dev and non-dev dependency.

Instead, I'd like to propose we add an option to explicitly ignore dev dependencies, that works pretty much like a composer install --no-dev.

After a quick look it appears like it mostly comes down to ignoring require-dev in composer.json and packages-dev in composer.lock. Special care has to be taken to not separate the resulting graph into two disconnected components, but some basic unit tests should be able to cover this case fairly well.

Refs clue/graph-composer#2