Currently, @astrojs/image allows *importing* images from srcDir
only. Importing images from outside srcDir fails miserably *in dev
mode* and produces incorrect src.
This happens because `path.relative(fileURLToPath(config.srcDir), id)`
resolves to "../something" and when joined with '/@astroimage' cancels
it out (`join('/@astroimage', '../../something')` => `'/something'`).
Rework /@astroimage URL scheme to be similar to "/@fs/" scheme—always
export absolute path to the target file.
