Automatically aggregate build products in one, consistent location

[jspahrsummers]

Originally from #8.

The intention is that a Carthage user should be able to drag-and-drop a framework built by Carthage into their project/workspace, and have it Just Work™ going forward—even when building for a different platform, or if the dependency version changes, or when a different build configuration is chosen.

This might mean using Xcode's BUILT_PRODUCTS_DIR, but that worries me because it can get deleted frequently, and doesn't let users commit binaries to source control (if desired).

It might be easier to use a common, top-level subfolder of the application's repository. Users who don't want to commit the binaries can ignore this folder. However, this might require adding some search paths to their project settings.

[mdiep]

However, this might require adding some search paths to their project settings.

@robrix suggested generating an xcconfig, which seems like a decent idea.

[jspahrsummers]

I agree, if it becomes necessary. It'd be nice to avoid any need for custom settings at all, though.

[jspahrsummers]

This issue also encapsulates bringing all dependencies up to the shared folder at the application level, so this is greater in scope than simply #12 (comment).