cleanups in prep for new script-only code

no behavioral changes, just lining up the test harness to make the next
commit cleaner
make it possible to use on script-only packages

These packages have no modules (e.g. setup.py has packages=[]), so they
can't import a MODULE/_version.py . Instead, their setup.py's
"build_scripts" command has been overridden to interpolate a version
string into a generated scripts= file.

To use versioneer on script-only packages, set versionfile_build=None