Allow full no externals

Tried to put: `externals:[]` in the config file doesn't works, still considering all dependencies of the `package.json` as external. It would be great to have an option to fully package all without requiring a `package.json` / `node_modules`

Start by tracing how the tsup config interprets externals:[] and how package.json dependencies are discovered. Add a regression test for a project without package.json or node_modules, and verify that an empty externals list bundles all dependencies rather than treating them as external.

Written by the indexing model from the issue text.