This addresses the feature request to stage dependencies in sysroots
from a couple years back:

    https://mail.gnome.org/archives/buildstream-list/2018-August/msg00009.html

This was actually deadcode, since node.validate_keys() was called
on the configure dictionary without the legacy command steps. If any
element was using the legacy commands, they would have been met with
a load time error anyway.

This commit also updates the cache key test, since removing these
legacy commands affects BuildElement internally in such a way as
to affect the cache keys.

Instead of relying on Element.search(), use Element.configure_dependencies() to
configure the layout.

Summary of changes:

  * scriptelement.py:

    Change ScriptElement.layout_add() API to take an Element instead of an element name,
    this is now not optional (one cannot specify a `None` element).

    This is an API breaking change

  * plugins/elements/script.py:

    Implement Element.configure_dependencies() in order to call ScriptElement.layout_add().

    This is a breaking YAML format change.

  * tests/integration: Script integration tests updated to use the new YAML format

  * tests/cachekey: Updated for `script` element changes

This patch adds a new test plugin which implements
Element.configure_dependencies() in order to offer better flexibility for
testing overlaps.

Newly added tests:

  * Test overlap warnings and errors when staging elsewhere than
    in the sandbox root.

  * Test unstaged files failure modes when staging elsewhere than
    in the sandbox root.

  * Test various overlap behaviors of OverlapAction, when different
    calls to Element.stage_dependency_artifacts() cause overlaps to
    occur after staging files into separate directories.