Initial support for manifest build

[glguy]

No description provided.

[kquick]

Lots of duplicated if clauses above. One possible alternative:

for attr in [ 'manifest', 'model', ''data', 'nodegroups' ]:
    if attr in step:
        path = step[attr]
        dirname = Path(f'{self.next_fresh():02}_{attr}')
        subdir = to_path.parent.joinpath(dirname)
        subdir.mkdir(parents=True, exist_ok=False)
        topath = subdir.joinpath(Path(path).name)
        # nodegroups is different for the following two; if that's intended, use if attr == "nodegroups" just here...
        getattr(self, attr)(base_path.joinpath(path), topath))
        step[attr] = str(dirname.joinpath(path).name)
        break   # [optional]

[glguy]

I agree that there's a bit of duplication here. I'm hesitant to try and factor out more of the commonality because I think the future holds more diversity in ingestion steps (if there's new ones to come) than less, and I don't want to make the code more complicated just to try and eliminate some of the duplication.

[kquick]

A small python idiom comment, but otherwise looks good to me.