Add mutual exclusivity validator for source vs legacy fields #1207

Context

The new SourceResolver data model is complete in _resolver.py (PRs #1228, #1235). Once the source field is activated on PackageSettings and VariantInfo (Phase 1 — #1291), we need validation to prevent users from mixing old and new config styles.

The design proposal (Migration section) states:

Top-level and variant-specific source settings are mutually exclusive with download_source, resolver_dist, wheel_server_url, and pre_build settings. It is an error to combine the new source settings with any of the old settings.

Parent epic: #1254 (Phase 1)

Problem

Without validation, a user can set both source and legacy fields in the same YAML file without error:

source:
  provider: pypi-sdist
  index_url: https://custom.example/simple
resolver_dist:
  sdist_server_url: https://other.example/simple
  include_wheels: true

The new source field would silently take precedence and the legacy resolver_dist is ignored. This is confusing and can lead to configuration mistakes that are hard to debug.

Acceptance criteria

• Setting source alongside download_source, resolver_dist, wheel_server_url, or pre_built raises a clear validation error
• Validation works at both package level and variant level
• Existing configs (legacy-only or new-only) continue to load without errors
• Error message clearly tells the user which fields conflict

Related

• Part of Activate source field on PackageSettings/VariantInfo and add source_resolver PBI property #1291 (Phase 1: Activate source field + PBI property)