Add API for composing lists in yaml nodes

Background

As part of #1061 we need to perform composition between lists like so:

# bar.bst

kind: autotools
dependencies:
  (>):
    - foo-lib.bst

# project.conf

elements:
  autotools:
    dependencies:
      - autotools.bst

These lists are expected to be composed into this:

  - autotools.bst
  - foo-lib.bst

This means we need to do something along the lines of:

default_deps = _yaml.node_get(project_conf, list, "dependencies")
deps = _yaml.node_get(element, list, "dependencies")
_yaml.composite(default_deps, deps)

But this will not work, because _yaml.composite() will only deal with _yaml.Nodes.

Task description

We should add some form of an API to allow doing this - I can see either of these things working:

• _yaml.composite() learns to deal with plain lists - the problem here is that we'd struggle providing provenance data, and the behavior of composite(list, list) isn't obvious (although !1601 (merged) will probably make that "safe append", at least for dependencies).
• _yaml.get_node() returns proper _yaml.Nodes when type=_yaml.Node for lists - this feels a bit more reasonable, but I'm likely overlooking something :)

Acceptance Criteria

We should be able to compose lists without creating naughty synthetic nodes.

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