Give info to the user about the distribution where the extension can be found.
env directory dropped entirely, as all the dependencies are handled by setuptools now, selected according to the Python version. I like the Makefile now...
It happens e.g. running 'pgxn check .' in a distribution dir.
I was giving up on the command, before discovering that all the problems found were to blame to Debian bug #554166. Added command documentation. Also command mixins refactored, with the addition of WithSpecLocal and WithSudo mixins and cleaning up of epilog passing.