avoid duplicate imports in generated versioneer.py

This moves all the imports from the individual sources files (src/*.py
and src/git/*.py) into one place: src/header.py . We can't usefully run
pyflakes against the individual source files, but we *can* run it
against the concatenated output versioneer.py after building it with
"setup.py make_versioneer".

Thanks to @ceball for the catch.

closes #37