`convert {apkbuild, gem, python}`: make concurrent generation pipeline

[Dentrax]

Melange currently fetch dependencies in order then bulk create all of the manifests. This resulting longer build times (10 to 30m, depends on dep count).

I'd be nice to create a concurrent worker pipeline that do fetch, parse, generate, write, respectively.

By taking advantage of concurrency, we can aim to generate lots of packages just under a minute.