skeleton pypi: make sure there is always a non-empty filename
skeleton pypi: make sure there is always a non-empty filename

This gives a better error message for packages that have external source links
that aren't valid, like pywin32.

Fixes #591.

dan-blanchard/conda-build,sandhujasmine/conda-build,ilastik/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,mwcraig/conda-build,dan-blanchard/conda-build,dan-blanchard/conda-build,mwcraig/conda-build,ilastik/conda-build,rmcgibbo/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,mwcraig/conda-build,ilastik/conda-build