Revert "Package managers cannot handle nested optionalDependencies"

This partially reverts commit 51e56f994c.

Ignoring plug'n'play, it looks like yarn and pnpm do get the
filesystem layout right for this scenario, which is good news.

It also looks like npm 10 will get it right too.

npm/darwin-x64/package.json

@@ -13,6 +13,9 @@
     "url": "https://opencollective.com/libvips"
   },
   "preferUnplugged": true,
+  "optionalDependencies": {
+    "@sharpen/sharp-libvips-darwin-x64": "0.0.1-alpha.1"
+  },
   "files": [
     "lib"
   ],