builder: use `.dart_tool/firebase_functions` – should always map to the package name

[kevmoo]

The convention for .dart_tool is to namespace things by the package that creates them.

If we are publishing this code as pkg:firebase_functions then that should be the name!

[Ehesp]

Did not know that was a convention!

[Ehesp]

Note this will also need changing the CLI: firebase/firebase-tools@main...invertase:firebase-tools:@invertase/dart#diff-453a0be1ee386185c4b45676af86e00bbde9a686a3c6577e8051cf6117c293e7R86