be explicit about extensions when importing lighthouse modules

[connorjclark]

Before lighthouse can use the new exports feature of package.json, we need to fix some imports here. Specifying the extension is considered a best practice for a few reasons:

1. the node module resolution algorithm is pretty complex, and optional extensions are part of the reason why
2. ES modules running in browsers don't necessarily require an extension, but imports won't be rewritten to try with an implied js extension (the browser will only request of the server exactly the import string). Technically one could leave out extensions and configure a server to resolve files correctly / serve JS files without an extension, but obviously no one does that. So browser code is always going to have imports ending with .js. All that to say, the consistency is nice across JS environments.

[jburger424]

CC: @avparmar