Curate output

[FFY00]

Right now we do not care much about the output. I wanted to pay a bit more attention to that.

I propose to keep the output limited to build backend output, this means swallowing the ensurepip output, and to curated messages from us (build). Our own messages should be clear that come from us, either by having an identifiable prefix or a specific style. Our messages should also be pretty limited, keeping the output clean. If we see need for debug messages we can offer that under a --debug flag.

For our messages, I propose adding a build.print_info method that could be overwritten, in warnings.showwarning fashion. The default implementation would use the logging module, but we would overwrite it in __main__ to use a colorized raw print.

[pganssle]

It might be a good idea to be semi-consistent with pip and tox here, with different grades of verbosity? -v, -vv and -vvv

[FFY00]

Is build complex enough to warrant that? It is always something that could be expanded anyway.

[gaborbernat]

This project calls virtualenv on python 2 and pip everywhere, so if it's matching will be easy to forward flags to virtualenv and pip😎