element.py: Adding new configure_dependencies() public API method

This patch implements the essentials of the proposal to extend the
dependency attributes:

   https://lists.apache.org/thread.html/r850aeb270200daade44261f16dbad403bf762e553b89dcafa0848fe7%40%3Cdev.buildstream.apache.org%3E

And essentially this will obsolete issue #931 by providing a more
suitable replacement for Element.search().

Summary of changes:

  * _loader/loadelement.pyx: The Dependency object now loads the `config` node,
    and raises an error if the `config` node is specified on a runtime-only
    dependency.

  * element.py: Created the new Element.configure_dependencies() virtual method.

    If elements implement this method, then a list of all dependencies are
    collected at element construction time and given to the implementing
    (depending) element.

    If a build dependency has more than one `config` specified, then
    it will be given to the Element twice, and if there is no `config`
    specified, then the tuple will still be given to the element with
    a Null `config`.

    Elements are provided via a new DependencyConfiguration type.

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