Allow alternatives

[jayvdb]

I wouldnt mind the check that all deps are installed correctly, if it was fast, and the delay was relative to the number of dependencies of the executable, and not relative to the number of packages installed.

I suspect it could be done much better than setuptools is doing it, as I know setuptools is iterating over the entire package collection multiple times. But I am reasonably sure a fix to setuptools isnt possible. I've just found @dephell which appears to be a very sane approach, and a responsive maintainer, and could probably be used to achieve the same while still being quite fast.

But, if the executable script was installed by an OS distro packager, like rpm or dpkg etc, it can verify that the files and dependencies are correctly installed (i.e. the user hasnt blitz their OS by using sudo pip install ..).

Ergo, it would be nice to have a few implementations, so users can choose what suits their needs best.