Extract pubspec.yaml from the package archive instead of the API endpoint before analysis SDK selection.

[isoos]

Original issue reported: #7267

The root cause was a stale cache: upload happened on a different runtimeVersion than the analysis instance, and post-upload cache purge does not happen on other runtimes. The analysis started with reading the stale-cached API endpoint, failing to access the version. The proper long-term solution is to update the analysis and not use the cached endpoint.

[isoos]

I think the issue was now fixed, and the potential further step (as implemented in #8862) is not really needed. Closing as completed.