Make project pull order insensitive

[honnibal]

spacy project pull was simply going through the commands in the order they're defined in the file, which means if you put an earlier-running command lower down, your pulls wouldn't work. In short examples it feels natural that of course, the earlier thing is earlier in the file. But when you add stuff over time, or have a template, you can easily just not have them in order. This already bit me when training the models, so I figured I'd better fix it.

The "fancy" approach to this sort of thing is to define the input/output dependencies as a graph, and then do a topological sort. I'm using a while loop because it gets the same result and the code is much clearer (imo). What I'm doing would even work if we wanted to download the things in parallel...In that case we would just use a thread or an async library to do the work, the outer loop would be the same. Maybe worth remembering if we want to support parallelism in spacy project run.

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