sysutils/doctl: Update to v1.28.0 and use Go Modules [1]

[1] - Upstream switched from dependencies to modules in 034d856 (2019-06-20)

• No need to set GO_PKGNAME as it is redundant with modules
• Switching to modules fixes the build with upcoming go1.13

PR: 240073
Submitted by: dg@syrec.org [1]
Sponsored by: DigitalOcean [2]

[2] DigitalOcean provided VM credit to test doctl