ok now it works

NuGet/PackageSourceMapping.cs

@@ -0,0 +1,16 @@
+using System.Collections.Immutable;
+
+namespace NuGet;
+
+public class PackageSourceMapping(ImmutableArray<PackageSource> sources)
+{
+    private readonly ImmutableDictionary<string, Task<NuGetClient>> _clients = sources.Select(b =>
+            new KeyValuePair<string, Task<NuGetClient>>(b.Pattern,
+                NuGetClient.CreateFromIndexUrlAsync(b.Url)))
+        .ToImmutableDictionary();
+
+    public Task<NuGetClient> GetClientAsync(string packageId) =>
+        _clients.FirstOrDefault(b => packageId.StartsWith(b.Key)).Value;
+}
+
+public record PackageSource(string Pattern, string Url);