Yes, I run -current so I update packages frequently.
After tedu's comment here I installed 7.5 in a VM and could build and run my Go software in it successfully, so I figured there were some Go build or config files or something somewhere getting in the way. I pkg_delete'd go, removed every Go build/config file I could find, and re-added Go with pkg_add. Now I can build apps with the previously mentioned modules; everything works fine now.
After tedu's comment here I installed 7.5 in a VM and could build and run my Go software in it successfully, so I figured there were some Go build or config files or something somewhere getting in the way. I pkg_delete'd go, removed every Go build/config file I could find, and re-added Go with pkg_add. Now I can build apps with the previously mentioned modules; everything works fine now.
Thank you for the responses. :-)