os/exec: use filepath.Base in Command

filepath.Base covers all scenarios
(for example paths like d:hello.txt)
on windows

LGTM=iant, bradfitz
R=golang-codereviews, iant, bradfitz
CC=golang-codereviews
https://golang.org/cl/59740050

src/pkg/go/build/deps_test.go

@@ -125,7 +125,7 @@ var pkgDeps = map[string][]string{
 	"os":            {"L1", "os", "syscall", "time"},
 	"path/filepath": {"L2", "os", "syscall"},
 	"io/ioutil":     {"L2", "os", "path/filepath", "time"},
-	"os/exec":       {"L2", "os", "syscall"},
+	"os/exec":       {"L2", "os", "path/filepath", "syscall"},
 	"os/signal":     {"L2", "os", "syscall"},
 
 	// OS enables basic operating system functionality,

src/pkg/os/exec/exec.go

@@ -12,6 +12,7 @@ import (
 	"errors"
 	"io"
 	"os"
+	"path/filepath"
 	"strconv"
 	"sync"
 	"syscall"
@@ -111,7 +112,7 @@ func Command(name string, arg ...string) *Cmd {
 		Path: name,
 		Args: append([]string{name}, arg...),
 	}
-	if !containsPathSeparator(name) {
+	if filepath.Base(name) == name {
 		if lp, err := LookPath(name); err != nil {
 			cmd.lookPathErr = err
 		} else {
@@ -121,15 +122,6 @@ func Command(name string, arg ...string) *Cmd {
 	return cmd
 }
 
-func containsPathSeparator(s string) bool {
-	for i := 0; i < len(s); i++ {
-		if os.IsPathSeparator(s[i]) {
-			return true
-		}
-	}
-	return false
-}
-
 // interfaceEqual protects against panics from doing equality tests on
 // two interfaces with non-comparable underlying types.
 func interfaceEqual(a, b interface{}) bool {