Draft: shared: Always reinstall missing shared parts
requested to merge xavier_thompson/slapos.recipe.build:fix/shared-always-install-if-missing into master
Ensure shared parts are always reinstalled when the location is missing, even if buildout calls update because the part was already installed and no option changed. This mirrors buildout's behavior to call install when the buildout_installed path is missing regardless of the options; in the case of shared parts buildout_installed is reported as empty, to ensure buildout will never uninstall the path since it may be shared.