src: map file symlinks for old node versions

[flakey5]

Old Node versions have file symlinks pointing to them, but the worker wasn't handling this (i.e. for https://nodejs.org/dist/v0.1.99/ there should be a node-v0.1.99.tar.gz file but there's not).

This:

• Makes a new constants file fileSymlinks.json that holds whatever files need symlinks. The key is the symlink location and the value is the actual location of the file
• Makes the build-r2-symlinks.js script read fileSymlinks.json and include them in the cached directory listings
• Makes the R2 provider handle these symlinks

[flakey5]

We're still well under the 10mb limit for a worker but since this file is only going to grow, I think we should continue looking at something like #159

[ovflowd]

Agreed :)

[ovflowd]

Can this be cleaned up a bit? Maybe instead if else, we can have different loops just to separate concerns. Performance here should not be a driving factor.

[flakey5]

I'm not really sure separating it into two different loops would look much better,

for (const file of Object.keys(fileSymlinks)) {
  const directory = `${dirname(file)}/`

  if (!(directory in cachedDirectories)) {
    continue;
  }

  const actualFile = await headFile(client, fileSymlinks[file]);

  cachedDirectories[directory].files.push({
    ...actualFile,
    name: basename(file)
  })
}

for (const file of Object.keys(fileSymlinks)) {
  const directory = `${dirname(file)}/`

  if (!(directory in cachedDirectories)) {
    continue;
  }

  const actualFile = await headFile(client, fileSymlinks[file]);

  const contents = await listDirectory(client, directory);
  contents.files.push({
    ...actualFile,
    name: basename(file),
  })

  cachedDirectories[directory] = contents;
}