Go ports: cleanup, finish transition to USES=go started with r505321 (cat. audio-mail)
- Remove custom build/install targets left in place after r505321
- Switch to the new GO_TARGET tuple syntax introduced in r512001
- Switch to go:modules when upstream already uses them
Reviewed by: tobik
Approved by: araujo (mentor), portmgr (adamw)
Differential Revision: https://reviews.freebsd.org/D21741