Prefetch optional macOS extension caches alongside PHP installation

__tests__/extension-packs.test.ts

@@ -0,0 +1,36 @@
+import {extensionPacks} from '../src/extensions';
+
+describe('Optional macOS extension archives', () => {
+  it('selects each requested pack once', async () => {
+    expect(
+      await extensionPacks(
+        'imagick, mongodb, memcached, imagick, redis',
+        '8.4',
+        'darwin'
+      )
+    ).toEqual(['imagick', 'mongodb', 'memcached']);
+  });
+
+  it.each(['', 'redis, yaml', ':imagick', 'imagick, :imagick'])(
+    'does not download unused or disabled packs: %s',
+    async input => {
+      expect(await extensionPacks(input, '8.4', 'darwin')).toEqual([]);
+    }
+  );
+
+  it.each(['imagick-3.8.1', 'mongodb-beta', 'memcached-github.com/a/b@main'])(
+    'preserves explicit version/source installation: %s',
+    async input => {
+      expect(await extensionPacks(input, '8.4', 'darwin')).toEqual([]);
+    }
+  );
+
+  it('keeps legacy PHP and other platforms on their existing paths', async () => {
+    for (const version of ['5.4', '7.4', '8.1', '8.6']) {
+      expect(await extensionPacks('imagick', version, 'darwin')).toEqual([]);
+    }
+    for (const os of ['linux', 'win32']) {
+      expect(await extensionPacks('imagick', '8.4', os)).toEqual([]);
+    }
+  });
+});