cmd/go: fix 'go vet' of package with external tests

For example, fixes 'go vet syscall', which has source
files in package syscall_test.

Fixes #8511.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant
https://golang.org/cl/152220044

src/cmd/go/test.bash

@@ -1085,6 +1085,20 @@ fi
 unset GOPATH
 rm -rf $d
 
+TEST go vet with external tests
+d=$(mktemp -d -t testgoXXX)
+export GOPATH=$(pwd)/testdata
+if ./testgo vet vetpkg >$d/err 2>&1; then
+	echo "go vet vetpkg passes incorrectly"
+	ok=false
+elif ! grep -q 'missing argument for Printf' $d/err; then
+	echo "go vet vetpkg did not find missing argument for Printf"
+	cat $d/err
+	ok=false
+fi
+unset GOPATH
+rm -rf $d
+
 # clean up
 if $started; then stop; fi
 rm -rf testdata/bin testdata/bin1

src/cmd/go/testdata/src/vetpkg/a_test.go

+package p_test

src/cmd/go/testdata/src/vetpkg/b.go

+package p
+
+import "fmt"
+
+func f() {
+	fmt.Printf("%d")
+}

src/cmd/go/vet.go

@@ -4,6 +4,8 @@
 
 package main
 
+import "path/filepath"
+
 func init() {
 	addBuildFlagsNX(cmdVet)
 }
@@ -28,10 +30,21 @@ See also: go fmt, go fix.
 }
 
 func runVet(cmd *Command, args []string) {
-	for _, pkg := range packages(args) {
-		// Use pkg.gofiles instead of pkg.Dir so that
-		// the command only applies to this package,
-		// not to packages in subdirectories.
-		run(tool("vet"), relPaths(stringList(pkg.gofiles, pkg.sfiles)))
+	for _, p := range packages(args) {
+		// Vet expects to be given a set of files all from the same package.
+		// Run once for package p and once for package p_test.
+		if len(p.GoFiles)+len(p.CgoFiles)+len(p.TestGoFiles) > 0 {
+			runVetFiles(p, stringList(p.GoFiles, p.CgoFiles, p.TestGoFiles, p.SFiles))
+		}
+		if len(p.XTestGoFiles) > 0 {
+			runVetFiles(p, stringList(p.XTestGoFiles))
+		}
+	}
+}
+
+func runVetFiles(p *Package, files []string) {
+	for i := range files {
+		files[i] = filepath.Join(p.Dir, files[i])
 	}
+	run(tool("vet"), relPaths(files))
 }