Make it possible to pass `--sdist <path-to-sdist>` to avoid (re)building an sdist

[pradyunsg]

This would be useful for development workflows where an sdist has been generated already -- instead of rebuilding the sdist in check-manifest again, it would make workflows quicker if it were possible to pass check-manifest an argument to use an already-created sdist.

[mgedmin]

Thing is, check-manifest builds an sdist twice:

• once directly in the source tree
• once in a temporary directory with a clean copy of the source files

and then compares both to each other, and to the list of files in the VCS.

When the user provides an already-built sdist, should it be considered a direct one, or a clean one, or should there be two command line options?