buildtag prints buildtags which are used in specified packages.
$ go install github.com/gostaticanalysis/buildtag/cmd/buildtag@latest
$ buildtag golang.org/x/net/context
!go1.7 /Users/tenntenn/go/pkg/mod/golang.org/x/[email protected]/context/context_test.go
go1.7 /Users/tenntenn/go/pkg/mod/golang.org/x/[email protected]/context/go17.go
go1.9 /Users/tenntenn/go/pkg/mod/golang.org/x/[email protected]/context/go19.go
!go1.7 /Users/tenntenn/go/pkg/mod/golang.org/x/[email protected]/context/pre_go17.go
!go1.9 /Users/tenntenn/go/pkg/mod/golang.org/x/[email protected]/context/pre_go19.go
$ buildtag -tag "^go1\.*" golang.org/x/net/context
/Users/tenntenn/go/pkg/mod/golang.org/x/[email protected]/context/go17.go:go1.7
/Users/tenntenn/go/pkg/mod/golang.org/x/[email protected]/context/go19.go:go1.9
$ buildtag -f="{{.Constraint}}" golang.org/x/net/context | sort | uniq
!go1.7
!go1.9
go1.7
go1.9