build: add -dlgo flag in ci.go (#21824)

This new flag downloads a known version of Go and builds with it. This
is meant for environments where we can't easily upgrade the installed Go
version.

* .travis.yml: remove install step for PR test builders

We added this step originally to avoid re-building everything
for every test. go test has become much smarter in recent go
releases, so we no longer need to install anything here.

internal/build/util.go

@@ -20,6 +20,8 @@ import (
 	"bytes"
 	"flag"
 	"fmt"
+	"go/parser"
+	"go/token"
 	"io"
 	"io/ioutil"
 	"log"
@@ -152,3 +154,28 @@ func UploadSFTP(identityFile, host, dir string, files []string) error {
 	stdin.Close()
 	return sftp.Wait()
 }
+
+// FindMainPackages finds all 'main' packages in the given directory and returns their
+// package paths.
+func FindMainPackages(dir string) []string {
+	var commands []string
+	cmds, err := ioutil.ReadDir(dir)
+	if err != nil {
+		log.Fatal(err)
+	}
+	for _, cmd := range cmds {
+		pkgdir := filepath.Join(dir, cmd.Name())
+		pkgs, err := parser.ParseDir(token.NewFileSet(), pkgdir, nil, parser.PackageClauseOnly)
+		if err != nil {
+			log.Fatal(err)
+		}
+		for name := range pkgs {
+			if name == "main" {
+				path := "./" + filepath.ToSlash(pkgdir)
+				commands = append(commands, path)
+				break
+			}
+		}
+	}
+	return commands
+}