Support for multi module gradle builds

[lnavarette]

This change partially resolves #663

High level overview of the changes:

• The BuildSystem already had methods for getting main/test source roots. I have updated a few of the standard ProjectContributors to use that so that I could extend the BuildSystem to be the source of truth for whether it's a single or multi module build.
• I extend the BuildSystem to support configuration of a module (other than root module) to hold the application, and it now provides a getApplicationModuleRoot so that ProjectContributors can grab that if they want to put files in the application module - regardless of whether it's a single or multi project build.
• I didn't update any of the factories to support this automatically, I took a direction similar to how the kotlin dialect was implemented, which requires a ProjectDescriptionCustomizer to inject the BuildSystem implementation
• I only implemented the changes for the gradle build system as I have more familiarity there. I'm happy to do the work to add the same for maven.

I'd like feedback on:

• if BuildSystem was the "correct" thing to extend to add this functionality or if there may be a better place. It seemed clean enough but in a few places I needed to introduce it as a dependency for another class.
• from a testing POV, what would you want to see to be confident these changes are working as expected?
• from a feature completeness POV - is the approach of requiring a ProjectDescriptionCustomizer acceptable?

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[tinesoft]

Any progress on that PR/ topic @wilkinsona, @snicoll ?

This would be a great addition to Spring Initializr.

I'll be interested in contributing as well, if needed.

[wilkinsona]

Unfortunately not, @tinesoft. We have a number of other things on our plates at the moment which are of higher priority.