Rewrite relative gleam imports in ffi code to resolve correctly.

[hayleigh-dot-dev]

The vite-gleam plugin for site lets one write javascript code that can import relative gleam files directly, for example import { main } from './app.gleam'.

We should add similar support so that users could do both of the following:

• Resolve relative gleam imports such as ./app.gleam to the compiled ./app.mjs path.
• Resolve bare gleam imports such as lustre/element.gleam to the relative path to the compiled gleam package ../lustre/lustre/element.mjs (assuming the js file importing this module was in the root of the app's source, adjust accordingly for nested ffi files supported by Copy nested ffi files into build/ #55)