Ensure backward compatibility for plugins

This commit ensures backward compatibility for plugins

Signed-off-by: Rohan Devasthale <rdevasth@redhat.com>

Author: rd4398

src/fromager/sources.py

@@ -135,22 +135,52 @@ def resolve_source_all(
     """Return list of (URL, version) for all matching source versions.
 
     Returns list sorted by version (highest first).
+
+    Supports both old and new plugin interfaces:
+    - New plugins implement resolve_source_all() returning list
+    - Old plugins implement resolve_source() returning single tuple (backward compat)
     """
     constraint = ctx.constraints.get_constraint(req.name)
     logger.debug(
         f"resolving requirement {req} using {sdist_server_url} with constraint {constraint}"
     )
 
     try:
-        resolver_results = overrides.find_and_invoke(
-            req.name,
-            "resolve_source",
-            default_resolve_source_all,
-            ctx=ctx,
-            req=req,
-            sdist_server_url=sdist_server_url,
-            req_type=req_type,
-        )
+        # Check for new plugin hook first (returns list)
+        new_plugin = overrides.find_override_method(req.name, "resolve_source_all")
+        if new_plugin:
+            resolver_results = overrides.find_and_invoke(
+                req.name,
+                "resolve_source_all",
+                default_resolve_source_all,
+                ctx=ctx,
+                req=req,
+                sdist_server_url=sdist_server_url,
+                req_type=req_type,
+            )
+        else:
+            # Fall back to old plugin hook (returns single tuple for backward compat)
+            old_plugin = overrides.find_override_method(req.name, "resolve_source")
+            if old_plugin:
+                single_result = overrides.find_and_invoke(
+                    req.name,
+                    "resolve_source",
+                    default_resolve_source,
+                    ctx=ctx,
+                    req=req,
+                    sdist_server_url=sdist_server_url,
+                    req_type=req_type,
+                )
+                # Wrap old plugin result in list
+                resolver_results = [single_result]
+            else:
+                # No plugin, use default
+                resolver_results = default_resolve_source_all(
+                    ctx=ctx,
+                    req=req,
+                    sdist_server_url=sdist_server_url,
+                    req_type=req_type,
+                )
     except (
         resolvelib.InconsistentCandidate,
         resolvelib.RequirementsConflicted,