Support cross-junction elements in Element.search()

Background

Follow-up from !998 (comment 144601678).

Now that both the CLI and element declarations support JUNCTION:ELEMENT style to refer to cross-junction elements inline, it might also be nice to support it via Element.search().

Original comment for reference:

One more thing which came to mind while working on creating a bootable image...

Some element implementations (like x86Image for example, or script elements), expose configurations which allow specifying an element name, the implementations later use Element.search() to obtain the dependency specified by the user.

I think it would be very useful to automatically support cross junction elements specified with a : in Element.search().

Task description

• Element.search() works with cross-junction dependencies, with : as separator for names of the junction and the element.

Acceptance Criteria

Element.search() works with cross-junction dependencies, with : as separator for names of the junction and the element.

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