New "browse" UI

Also, separated out browse, ?meta, and ?module request handlers.

Fixes #82

modules/middleware/validateVersion.js

@@ -0,0 +1,49 @@
+import createPackageURL from '../utils/createPackageURL.js';
+import { getPackageConfig, resolveVersion } from '../utils/npm.js';
+
+function semverRedirect(req, res, newVersion) {
+  res
+    .set({
+      'Cache-Control': 'public, s-maxage=600, max-age=60', // 10 mins on CDN, 1 min on clients
+      'Cache-Tag': 'redirect, semver-redirect'
+    })
+    .redirect(
+      302,
+      createPackageURL(req.packageName, newVersion, req.filename, req.search)
+    );
+}
+
+/**
+ * Check the package version/tag in the URL and make sure it's good. Also
+ * fetch the package config and add it to req.packageConfig. Redirect to
+ * the resolved version number if necessary.
+ */
+export default async function validateVersion(req, res, next) {
+  const version = await resolveVersion(req.packageName, req.packageVersion);
+
+  if (!version) {
+    return res
+      .status(404)
+      .type('text')
+      .send(`Cannot find package ${req.packageSpec}`);
+  }
+
+  if (version !== req.packageVersion) {
+    return semverRedirect(req, res, version);
+  }
+
+  req.packageConfig = await getPackageConfig(
+    req.packageName,
+    req.packageVersion
+  );
+
+  if (!req.packageConfig) {
+    // TODO: Log why.
+    return res
+      .status(500)
+      .type('text')
+      .send(`Cannot get config for package ${req.packageSpec}`);
+  }
+
+  next();
+}