Render main.js to same directory, so it can use relative imports

[Rich-Harris]

At present, main.js is written to the destination directory (usually .sapper). That was so that we could write it to /tmp in Now, back before building was separated out from serving.

Writing it back to the same folder (as .main.rendered.js or something) would allow it to use relative imports, when necessary.

Might then be a bit weird that you can't do that with service-worker.js. Need to think about that, maybe in a separate issue.