Use astral-sh/versions as primary version provider (#802)

Closes: #777
Closes: #325

src/download/variant-selection.ts

@@ -0,0 +1,39 @@
+interface VariantAwareEntry {
+  variant?: string;
+}
+
+export function selectDefaultVariant<T extends VariantAwareEntry>(
+  entries: T[],
+  duplicateEntryDescription: string,
+): T {
+  const firstEntry = entries[0];
+  if (firstEntry === undefined) {
+    throw new Error("selectDefaultVariant requires at least one candidate.");
+  }
+
+  if (entries.length === 1) {
+    return firstEntry;
+  }
+
+  const defaultEntries = entries.filter((entry) =>
+    isDefaultVariant(entry.variant),
+  );
+  if (defaultEntries.length === 1) {
+    return defaultEntries[0];
+  }
+
+  throw new Error(
+    `${duplicateEntryDescription} with variants ${formatVariants(entries)}. setup-uv currently requires a single default variant for duplicate platform entries.`,
+  );
+}
+
+function isDefaultVariant(variant: string | undefined): boolean {
+  return variant === undefined || variant === "default";
+}
+
+function formatVariants<T extends VariantAwareEntry>(entries: T[]): string {
+  return entries
+    .map((entry) => entry.variant ?? "default")
+    .sort((left, right) => left.localeCompare(right))
+    .join(", ");
+}