build: verify and test new source resolver dispatch end-to-end #1403

Part of #1294 (Phase 8 of epic #1254). Branch: new-resolver-config.

What needs to happen

Verify src/fromager/commands/build.py end-to-end for source:-config packages. This should mostly work already since the command routes through sources.resolve_source() and sources.download_source(), which dispatch on pbi.source_resolver (Phases 2 and 3 are complete):

• Walk through a build of a source:-config package and confirm resolution and download go through the new dispatch.
• Add a regression test so the wiring stays in place.
• Only open a code change if the walkthrough finds a gap.

Tests

• Build of a source-config package drives resolution/download through the new dispatch

Notes

Smallest of the #1294 sub-issues (likely test-only). Independent of the others; can be worked on in any order.