% cat x.go
package main
import _ "cmd/api"
func main() {}
% GOTOOLCHAIN=go1.19 go build x.go
x.go:2:8: import "cmd/api" is a program, not an importable package
% GOTOOLCHAIN=go1.20 go build x.go
% GOTOOLCHAIN=go1.21.0 go build x.go
%