Hide sourcemaps in dynamic-imports

[rebane2001]

At the moment, it is possible to read the client sourcemap files like this:

fetch("http://localhost/__meteor__/dynamic-import/fetch", {
  "body": JSON.stringify({"imports":{"index.js.map":1}}),
  "method": "POST",
  "mode": "cors",
}).then(e => e.text()).then(console.log)

This is due to the meteor/dynamic-import package accessing the filesystem directly, as opposed to using staticFiles.

I mitigated it by adding a handler to Meteor that destroys the response when an invalid request is received. It is a pretty hacky solution, but it's the best I could come up with and it seems to work great, feel free to suggest a better one.