add _get_package_name

_get_package_name will return the package name by givan the full
package name (including version)
_get_package_name python-cinderclient-1.0.9.tar.gz
_get_package_name python-cinderclient-1.0.tar.gz
_get_package_name python-cinderclient-1.0.whel
return python-cinderclient