Refactor: populate source: settings early and unify resolver dispatch #1451

Follow-up to the structure discussion in #1414 (see Doug's review comment).

Problem

The source: feature (#1414) supports both the new settings and the legacy settings (resolver_dist, download_source, variant pre_built / wheel_server_url, plugin hooks) at the same time. As a result, every point in the pipeline that decides how to handle a package carries its own "is there a source: config?" branch, falling back to legacy behavior locally. There are about 15 of these branches today (get_source_provider(), download_source(), build_sdist(), download_prebuilt_wheel(), package resolve, find-updates, list-versions, list-overrides, download-sequence, the bootstrap resolver, and others).

Each branch is individually correct, but every new entry point has to remember the check. That is exactly how #1407, #1408 and #1413 happened — three separate fixes for commands that each missed the configured-resolver path.

Direction

Once packages have migrated to source: settings:

1. Populate the source: section at settings-load time. Packages with a config get their resolved (variant-merged) settings; packages without one get a synthetic default profile. Everything downstream can then rely on a valid source: existing.
2. Replace the per-site fallback branches with a uniform handler: instantiate the handler for the profile and call one consistent API (Strategy pattern), instead of if/elif/match dispatch in each layer.

Most of the ~15 branches exist only to serve the legacy path, so they become deletions rather than rewrites at that point.

Preconditions

• Legacy settings are deprecated (Warn users when legacy resolver settings are configured #1388) and downstream plugins/configs have moved to source:. hook-sdist / hook-prebuilt are the bridge profiles for plugins.
• Not before 0.103.0 ships — building the handler layer now would mean writing it to accommodate legacy semantics and building it again after legacy is removed.

References

• feat(resolver): add the declarative source: configuration #1414 — the source: feature and the review discussion
• Warn users when legacy resolver settings are configured #1388 — deprecation warnings for old settings
• Proposal: docs/proposals/new-resolver-config.md (Deprecations / Migration sections)