Every pipeline that gets added to the `Manifest` now need to have
a unique name by which it can be identified. The version 1 format
loader is changed so that the main pipeline that builds the tree
is always called `tree`. The build pipeline for it will be called
`build` and further recursive build pipelines `build-build`, where
the number of repetitions of `build` corresponds to their level of
nesting. An assembler, if it exists, will be added as `assembler`.
The `Manifest.__getitem__` helper is changed so it will first try
to access pipeline via its name and then fall back to an id based
search. NB: in the degenrate case of multiple pipelines that have
exactly the same `id`, i.e. same stages, with the same options and
same build pipeline, only the first one will be return; but only
the first one here will be built as well, so this is in practice
not a problem.
The formatter uses this helper to get the tree pipeline via its
name wherever it is needed.
This also adds an `__iter__` method `Manifest` to ease iterating
over just the pipeline values, a la `for pipeline in manifet`.
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