Don't allow timeouts to be registered when shutting down

Currently, a timeout that sets a timeout can cause loop.run to block forever.

Also, cleanup URL stitch with leading './'. The resulting URL was valid, but
since we use the URL as the module cache key, it's important to resolve various
representations of the same URL in the same way.

src/runtime/loop.zig

@@ -187,6 +187,12 @@ pub const Loop = struct {
     }
 
     pub fn timeout(self: *Self, nanoseconds: u63, callback_node: ?*CallbackNode) !usize {
+        if (self.stopping) {
+            // Prevents a timeout callback from creating a new timeout, which
+            // would make `loop.run` run forever.
+            return 0;
+        }
+
         const completion = try self.alloc.create(Completion);
         errdefer self.alloc.destroy(completion);
         completion.* = undefined;