Modify `external_package_dirs` behavior to copy dir name based on flag

[bolasim]

Is your feature request related to a problem? Please describe. Currently, when I use external_package_dirs, it copies what's inside the folder and I have to restructure the whole repo to get imports to work (by moving everything inside one directory that has the correct import paths as they would show up after things are copied to the truss).

However, often times I work with open source repos that use top level imports and moving that many things around is impractical, especially when I want to contribute the truss back to the repo to increase reach.

Describe the solution you'd like We can add a config called keep_dir_names that copies the directory names so that the imports in other directories that are copied work without modification and it's easier to avoid conflicts without changing everything else in the repo.