Make http(s) URIs their own module key (#495)

GenericUrl is a catch-all that uses URL.openConnection().
Since we now have special handling of HTTP urls, it makes more sense to
put it in its own module key.

pkl-server/src/main/kotlin/org/pkl/server/Server.kt

@@ -223,6 +223,7 @@ class Server(private val transport: MessageTransport, private val httpClient: Ht
     add(ModuleKeyFactories.modulePath(modulePathResolver))
     add(ModuleKeyFactories.pkg)
     add(ModuleKeyFactories.projectpackage)
+    add(ModuleKeyFactories.http)
     add(ModuleKeyFactories.genericUrl)
   }
 }