Integrate functionality from check-manifest

Originally reported by: freakboy3742 (Bitbucket: freakboy3742, GitHub: freakboy3742)

A common problem with Python packaging is creating and maintaining an accurate MANIFEST/MANIFEST.in file. It's easy to add a new submodule or piece of documentation, forget to include it in your MANIFEST, upload a release to PyPI, then have to issue a new point release to correct the problem.

check-manifest is a package that addresses this problem. If you run the command, it builds a source distribution and checks to see if there are files in the source directory that aren't in the package distribution; if there are missing files, it suggests MANIFEST.in rules that would correct the omission. It can also be used to generate an initial MANIFEST.in for a project.

In my opinion, check-manifest is sufficiently useful functionality that it warrants being part of setuptools itself.

• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/57

No setuptools files, tests, or entry points are named. Start by reviewing check-manifest's documented behavior and the setuptools repository structure to identify where source-distribution manifest handling belongs. Done would require an agreed integration scope and equivalent checks or manifest generation within setuptools.