Break middleware out into separate files

server/middleware/checkBlacklist.js

@@ -0,0 +1,14 @@
+/**
+ * Check the blacklist to see if we can serve files from this package.
+ */
+function checkBlacklist(blacklist) {
+  return function (req, res, next) {
+    if (blacklist.includes(req.packageName)) {
+      res.status(403).send(`Package ${req.packageName} is blacklisted`)
+    } else {
+      next()
+    }
+  }
+}
+
+module.exports = checkBlacklist