properly support private dependencies

currently, Depends inside Export represent the link-time dependencies of the exported module literally. this is suboptimal:

• the specification is a redundant subset of the Product's Depends, which is unnecessarily verbose, and harder to maintain and tool
• it actually doesn't work: for the product type to be easily switchable (say, choosing on the command line whether to build a library statically or dynamically), the declaration of the dependencies of the Product itself must be complete, irrespective of how the linker needs to be invoked to build it.
  • static libraries don't actually use their Depends, but forward them directly to their Export
  • dynamic libraries link against their Depends
    • there must be a flag to specify how these are forwarded to the Export
      • public dependencies on other dynamic libs (which become part of the library's link interface) are exported, so they are added as an -L/-l pair by the user
      • private dependencies on other dynamic libs also need to be exported, because some platforms (linux and bb10 at least) may need -rpath-link to resolve transitive dependencies
  • applications also export their dependencies on dlls (privately), as they must be known when launching the application (see QBS-583)

one possible approach is to "tag" dependencies with a property:

Product {
    Depends {
        name: "foo"
        //exported: false // the default
    }
    Depends {
        name: "bar"
        exported: true   // "export" is a reserved word
        // publicly: true   // alternative name, think "depends publicly"
    }
}

a probably better approach is to nest the exported dependencies into the exports themselves. this mixes well with products that export multiple sub-products, as we do in qt itself:

Product {
    // equivalent of QT_PRIVATE += multimedia
    Depends {
        name: "multimedia"
    }
    Export {
        name: "qt.location"
        // equivalent of QT += gui
        Depends {
            name: "qt.gui"
        }
    }
    Export {
        name: "qt.location_private"
        // equivalent of QT_FOR_PRIVATE += quick
        Depends {
            name: "qt.quick"
        }
    }
}

note that using this option would mean that the exported Modules would have to actually contain Export items to properly declare the type of their Depends.