MAINT: restructure the interface between the project class and the wheel builder

[dnicolodi]

This is an attempt at making the interface between the wheel builder class and the class representing the meson project a bit easier to understand. Ideally we should be able to just pass the manifest and the metadata to the wheel builder class.

[dnicolodi]

@rgommers This is a relatively large scale reorganization of the code, but I find the resulting code much easier to reason about. It is still work in progress, but I would like to know if you like the direction in which this goes before putting the final touches to it.

[dnicolodi]

Also, I've tried to split the changes in small self-standing commits. I think this makes the changes easier to review. However, if you prefer a big patch, that would be easier to generate because I don't need to make sure that the tests pass for each intermediate step.

[rgommers]

Thanks! This looks like a useful set of changes to me. Making this interface as clean as possible will be helpful to future maintenance. The individual commits are quite helpful for reviewing - I read the full diff first, but it's pretty hard to review that. Each individual commit looks pretty good.

[dnicolodi]

Great. I'll finish this then.

[dnicolodi]

This is now ready for review. There is room for further simplification, but this is already getting big.