all: convert to go:build directives

[sbinet]

as mentioned in golang/go#41184, Go will probably migrate to a more robust build constraints directive, as explained there:

• https://golang.org/s/go-build-design

we should prepare for that:

• for Go-1.17: add all //go:build directives
• for Go-1.18: remove all // +build directives

[rsc]

Please wait until Go 1.17 to add any //go:build directives.
Go 1.16 is only setting up for Go 1.17. Go 1.17 is the real change.

[sbinet]

Thanks for pointing this out.
I've modified the planned timeline.