separating the nix fixture from the __init__.py file to increase
readability and overview over the existing fixtures
Change-Id: I7a86cb729942e83a95b9eabbb09563822f3f9e54
Documenting fetchTree is an exercise in frustration because of the sheer
amount of stringly typed everything in it. I do not know which fields
exist without reading the entirety of libfetchers. However, we can write
something slightly perfunctory but at least perhaps somewhat helpful
documentation-wise.
Fixes: https://git.lix.systems/lix-project/lix/issues/609
Change-Id: I991391b53fcd69172dbc7efb9d384e62bc847b91