A 'clone' operation has an implicit understanding that it is expensive,
which is not the case of a 'copy' operation, which is more usually
a shallow copy.

Therefore renaming to 'clone'

This makes the 'Node' API public, and available for use directly for
plugins.

- _composite -> __composite (internal)
- composite -> _composite (BuildStream private)
- composite_under -> _composite_under (BuildStream private)
- get -> _get (internal)

This replaces the helper method by adding a 'get_provenance' on the node
directly

- Adapt all call sites
- Delay getting provenance wherever possible without major refactor

- Also take care of node_composite_move in the same way.
- Adapt all calling places

This is to mimic the 'dict.values()' interface.

- Adapt parts of the code calling 'node_items' but ignoring the
first value to use this instead

- Also add a convenience method 'safe_del' catching the exception
when we don't care if the value was there or not.

Also adaprt every part of the code calling it

Sometimes, we might want to have a list or a string, in which case, we
can now use 'get_node', which can return multiple types.

This method doesn't contain a 'default' value, as building a 'Node' from
it would be hard. We therefore just have a 'allow_none' flag that allows
returning 'None' when the value is not found.

- includes: use the new 'get_node' instead of trying multiple versions

- Adding 'get_sequence' on MappingNode to access sequences in a mapping
- Adding 'sequence_at' on SequenceNode to access sequences in a sequence
- Adding 'mapping_at' on SequenceNode to access mappings in a sequence

Using "*_at" in sequences allows us to quickly understand if we are
dealing with a sequence or a mapping.