Could not detect language for functions at functions

I came across this issue with a custom build process and pointing only to my dist folder.

There are three requirements for a successful build:

1. a firebase.json with functions.source pointing to a directory containing a package.json
2. The package.json containing a "main":"path-to-index.js" field.
3. A node engine specified in either the package.json or firebase.json

//firebase.json
{
  "functions": {
    "source": "./some-dir"
  }
}
//some-dir/package.json
{
  "main": "lib/index.js",
  "engines": {
    "node": "16"
  }
}