Fix ?meta listings

Fixes ?meta listings (and other requests) for packages with a root
"directory" entry.

Fixes #210

modules/__tests__/serveMetadata-test.js

@@ -0,0 +1,32 @@
+import request from 'supertest';
+
+import createServer from '../createServer.js';
+
+describe('A request for metadata', () => {
+  let server;
+  beforeEach(() => {
+    server = createServer();
+  });
+
+  it('returns 200', done => {
+    request(server)
+      .get('/react@16.8.6/?meta')
+      .end((err, res) => {
+        expect(res.statusCode).toBe(200);
+        expect(res.headers['content-type']).toMatch(/\bapplication\/json\b/);
+        done();
+      });
+  });
+
+  describe('with a package that includes a root "directory" entry', () => {
+    it('returns 200', done => {
+      request(server)
+        .get('/sinuous@0.12.9/?meta')
+        .end((err, res) => {
+          expect(res.statusCode).toBe(200);
+          expect(res.headers['content-type']).toMatch(/\bapplication\/json\b/);
+          done();
+        });
+    });
+  });
+});