New isolate for the SDK index in the search instance.

[isoos]

• part of SDK search index improvements (tracking issue) #8670
• on a local benchmark, this removed about 5-8% of the overall wall-clock latency with a single thread load, I think the effect may be more with the real traffic which will have higher concurrency
• This PR refactored PackageIndexHolder and SearchResultCombiner, but mostly kept them in place. I think we should probably merge those, and make other parts simpler (planned in a follow-up PR).
• note: we (still) lack the test coverage for the entrypoint/search.dart file, will address that in a follow-up PR