--packages external should bundle workspace members

[mattfysh]

What is the problem this feature would solve?

This would allow a more logical definition of what is an external package, and what should be included in a bundle

What is the feature you are proposing to solve the problem?

When using bun build --packages external any workspace members should not be externalized. An example is available here: https://github.com/mattfysh/bunpkgsext

packages/a depends on packages/b and lodash - when bundling A with --packages external it externalizes both B and lodash. Because package B is a workspace member and not a third party package, it should not be excluded from the bundle

This would also bring bun in line with esbuild

What alternatives have you considered?

1. Manually listing each node_module as --external -- too fussy and error prone
2. Writing a plugin to decide which packages to externalize