python: make API generation order irrelevant

[jskeet]

Currently, when a library contains multiple API paths with the same version (e.g. google/cloud/firestore/v1 and google/cloud/firestore/admin/v1) they both end up being generated into owl-bot-staging/v1, and the last one to be generated effectively takes priority for things like setup.py.

The simplest (though kludgy) way of fixing this is to generate APIs with the longest paths first - we always want the shortest path to take priority, per version.

We should figure out what we really want the files to look like in cases like this (the overwriting is awkward), and change generation accordingly, ideally while removing the idea of "staging" entirely.