Refactor: separate lookup from filter+decide stages

[rarkins]

Currently Renovate does lookup+filter+decide logic per-dependency. Instead, I'd prefer to:

• Flatten extracted dependencies into a single-dimensioned array
• Look up (via datasources) all dependencies simultaneously (but with rate limits)
• Once lookups are complete, perform the filter+decide step

Separating like this makes more logical sense, and will also make it easier in future in case we want to optimize such as:

• Caching the entire lookup result and re-using it if running multiple times in a row, or
• Querying a lookup cache with a single query (list of all packages) instead of one cache query per package

[rarkins]

Waiting on #6358 - we should have some default rate limit like 10 before we fully parallelise lookups.