RFE: Allow `bst show` to print list of dependencies

Background

I would like to build a tool, outside of BuildStream, that would print nice dot graphs of BuildStream elements. This would make it easier to visualize the graph easily. But, doing so requires knowing what are the direct dependencies of a given element.

This is not currently possible to achieve with bst show as it only prints a list of all elements, without any information about the dependencies. We can do this by reading the .bst files ourselves, but that is more error-prone, especially since one can have complicated yaml magic in the .bst files.

Task description

It would be good to add format strings for various kinds of dependencies: all, build and runtime.

Acceptance Criteria

It is possible to get a list of dependencies for given elements using the bst command line, and reconstruct the entire dependency graph using that information.

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