Make Invert more build system agnostic

Invert was originally built with just the Gradle build system in mind, but we've seen it working and providing values for other build systems as well. Most build systems seem to just leverage these primitives:

• Modules (Dependencies, Plugins, Configurations)
• Ownership

Note: We shouldn't "remove" any existing support for Gradle, or the ability to add onto it, but we're still early enough to be able to make that information pluggable and not alienate other usages.

Collection

Gradle Modules should just be one type of module type can be used. If we have a way of using a Gradle Plugin (or based on directory traversal) to determine projects and dependencies, it should be able to capture all we need.

Report

We have a dedicated section of the Invert report just to Gradle. We don't need that section when generating reports for other build systems.

This also makes me realize we need to separate out the Kotlin specific features. (Created https://github.com/square/invert/issues/28)

Start by mapping where Gradle-specific collection and reporting are implemented, then read issue #28 for the related Kotlin-specific separation. The work is done when modules and ownership are pluggable for other build systems while existing Gradle support remains intact and Gradle-only report sections are optional.