Dev module loader doesn't support live bindings

[Rich-Harris]

I thought we might be able to get away with this, since live bindings aren't likely to be an issue during SSR, but I was wrong — working on #229, I discovered that in order to make paths configurable at runtime (so that svelte-kit start continues to work with a build that defaults to the paths declared in svelte.config.js), we need to have live bindings. Otherwise

import { base, assets } from '$app/path';

will yield a pair of undefined paths.

I think I know how to implement it (replace all references to named imported bindings like foo with __import1.foo, replace all exports.foo = foo with Object.defineProperty(exports, 'foo', { get: () => foo }), it's just a bit of a nuisance