feat(resolver): add LegacyResolver for legacy resolution and download

[tiran]

Pull Request Description

What

Add LegacyResolver class that preserves the existing resolution and
download behavior as a typed resolver. When the source field is
eventually enabled on PackageSettings, packages without an explicit
typed resolver can use provider: legacy to keep the old behavior.

resolver_provider() replicates sources.get_source_provider(): reads
resolver_dist settings from per-package build info, invokes the
get_resolver_provider override hook, and applies the per-package
cooldown.

download() checks pbi.pre_built: when true, delegates to the
pre-built wheel downloader; otherwise invokes the download_source
override hook via overrides.find_and_invoke().

Why

Legacy provider will be used to gradually update to new resolver and downloader.

• PR follows CONTRIBUTING.md guidelines

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[mergify]

This pull request has merge conflicts that must be resolved before it can be merged.
@tiran please rebase your branch.