The importation into the United States of any alien for the purpose of prostitution, or for any other immoral purpose, is forbidden. Whoever shall, directly or indirectly, import, or attempt to import into the United States any alien for the purpose of prostitution or for any other immoral purpose, or shall hold or attempt to hold any alien for any such purpose in pursuance of such illegal importation, or shall keep, maintain, control, support, employ, or harbor in any house or other place, for the purpose of prostituti