entry_point into directory artifacts

Such as when source is copied into a directory using `copy_to_directory`.

Should it support https://github.com/aspect-build/bazel-lib/blob/main/docs/directory_path.md or similar?

Example: https://github.com/jbedard/rules_esbuild/tree/multi-dirs/examples/gen_entries

Start by reading docs/directory_path.md and the examples/gen_entries example linked in the issue. Determine the intended behavior for an entry_point that refers to directory artifacts copied with copy_to_directory, then document the decision and verify the resulting behavior for the linked multi-directory example.

Written by the indexing model from the issue text.