[FR]: Support `directory_path` for bundling

### What is the current behavior?

Currently entry-points need to be actual sources that can be referenced via labels. This is fine, but doesn't work when trying to e.g. bundle a file that originates from an npm dependency e.g. `//:node_modules/bla --> dist/index.mjs`.

### Describe the feature

`rules_js` generally makes use of `directory_path`, so it would be a canonical solution to respect those "labels" as well when bundling via Rollup.

The issue names rules_rollup, rules_js, Rollup, and directory_path but no files or tests. Start by tracing how bundling handles entry-point labels, then compare that behavior with the directory_path labels used by rules_js. Done means an npm dependency label such as //:node_modules/bla can be bundled to dist/index.mjs.

Written by the indexing model from the issue text.