reorg: move JS ops implementations to cli/js/ops/, part 2 (#4283)

Following JS ops were moved to separate files in cli/js/ops directory:
- io
- process
- worker_host
- web_worker
- plugins
- timers
- signal
- permissions

cli/js/net.ts

@@ -1,6 +1,6 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 import { EOF, Reader, Writer, Closer } from "./io.ts";
-import { read, write } from "./files.ts";
+import { read, write } from "./ops/io.ts";
 import { close } from "./ops/resources.ts";
 import { sendSync, sendAsync } from "./ops/dispatch_json.ts";