(Perhaps this deserves another thread, even though it is in the original post?)
I think that this question is no longer needed if a package is not presented as something different from a project, but as a special kind of project - characterized by the src folder with a jl file containing a module, matching the name of the package.
Under this view, the dilemma of “project or package” is like comparing e.g. “project with or without [compat] section”. The package structure gives the advantages that you mention, at the (minimal) cost of adding a structure that might be superfluous if you are not going to benefit from those advantages.