python_wheel can't repackage meta-packages #315

There are instances of "meta-packages" on PyPI that provide no code of their own but simply exist as a means of depending on other packages. The case I came across was typer-slim. These packages can't be repackaged into pex files by python_wheel because it expects outs to have a truthy value - passing outs = [] simply adds the name of the package to outs. It would be good for the default value of outs to be used only if it isn't explicitly passed as a parameter.