Wire source_resolver into resolver and download pipelines

[LalatenduMohanty]

PR #938 defines a model layer for declarative source configuration with provider classes, a RetrieveMethod enum, and discriminated union dispatch. This issue tracks wiring that model layer into the runtime so YAML-configured providers are used by the resolver and download pipelines.

Work items:

1. Modify default_resolver_provider() in resolver.py to check pbi.source_resolver before returning PyPIProvider
2. Modify default_download_source() in sources.py to detect git+https:// / git+ssh:// URLs and route to the git clone path
3. Add remove_dot_git field to GitOptions
4. Connect git_options (submodules, submodule_paths, remove_dot_git) to the git clone download path

Plugin precedence is preserved — overrides.find_and_invoke() calls the plugin hook first. The source_resolver check only runs when no plugin exists.

See: #937, #938, #936

[tiran]

Your ticket is a couple steps ahead. First the proposal needs to be approved. Next I'll update and push my configuration scaffolding. Then we can implement the logic and integrate it with the rest.