x/tools/internal/imports: zstdlib.go fails to generate with Go 1.20

[dmitshur]

Doing go generate golang.org/x/tools/internal/imports is fine with Go 1.19 and older, but fails when using Go 1.20:

$ go version
go version go1.20 darwin/arm64

$ go generate
2023/02/02 09:41:00 go command exited unsuccessfully: exit status 1
package cmd/api is not a main package
exit status 1
imports.go:5: running "go": exit status 1

The error output shown above was using following patch; the real error output is more inscrutable without it.

 cmd := exec.Command("go"+exeSuffix, "run", "cmd/api", "-contexts", "js-wasm", "syscall/js")
 out, err := cmd.Output()
-if err != nil {
+if ee := (*exec.ExitError)(nil); errors.As(err, &ee) {
+	log.Fatalf("go command exited unsuccessfully: %v\n%s", ee.ProcessState.String(), ee.Stderr)
+} else if err != nil {
 	log.Fatalln(err)
 }

The problem is that CL 453258 rewrote cmd/api to be a package rather than command, since it was more convenient for cmd/dist that way and cmd/api isn't really meant for external use.

It seems straightforward enough not to use of all cmd/api for the purposes of imports. I'll send a fix.

CC @rfindley, @bradfitz.

[gopherbot]

Change https://go.dev/cl/464715 mentions this issue: internal/imports: use go/packages instead of cmd/api to compute symbols