The rewrites for modern extension packaging had some regressions when

working off of a clean source tree and when processing lists of

entrypoints.
We had assumed the build/lib directory existed, but that wasn't a

guarantee at this point. This is now created when we need it.
We also assumed the extension's static directory existed, and broke if

it didn't. We now simply skip static media building in this case.
For the entrypoints, we now check if the result is a list or a string.

In either case, we convert to a string with an entrypoint per line, for

processing.

Successfully built rbintegrations and Power Pack packages.