Fix imports for Go 1.5+

Particularly important for 1.6+, since the deprecated imports have been
removed entirely.  Fixed by syncing with upstream stringer (includes
some other minor fixes/modifications).

stringer.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.5
+
 // Stringer is a tool to automate the creation of methods that satisfy the fmt.Stringer
 // interface. Given the name of a (signed or unsigned) integer type T that has constants
 // defined, stringer will create a new self-contained Go source file implementing
@@ -64,20 +66,18 @@ import (
 	"fmt"
 	"go/ast"
 	"go/build"
+	exact "go/constant"
 	"go/format"
+	"go/importer"
 	"go/parser"
 	"go/token"
+	"go/types"
 	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
 	"sort"
 	"strings"
-
-	"golang.org/x/tools/go/exact"
-	"golang.org/x/tools/go/types"
-
-	_ "golang.org/x/tools/go/gcimporter"
 )
 
 var (
@@ -90,7 +90,7 @@ var (
 func Usage() {
 	fmt.Fprintf(os.Stderr, "Usage of %s:\n", os.Args[0])
 	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T [directory]\n")
-	fmt.Fprintf(os.Stderr, "\tstringer [flags[ -type T files... # Must be a single package\n")
+	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T files... # Must be a single package\n")
 	fmt.Fprintf(os.Stderr, "For more information, see:\n")
 	fmt.Fprintf(os.Stderr, "\thttp://godoc.org/golang.org/x/tools/cmd/stringer\n")
 	fmt.Fprintf(os.Stderr, "Flags:\n")
@@ -129,7 +129,7 @@ func main() {
 	}
 
 	// Print the header and package clause.
-	g.Printf("// Code generated by \"stringer %s\"; DO NOT EDIT\n", strings.Join(os.Args[1:], " "))
+	g.Printf("// Code generated by \"enumer %s\"; DO NOT EDIT\n", strings.Join(os.Args[1:], " "))
 	g.Printf("\n")
 	g.Printf("package %s", g.pkg.name)
 	g.Printf("\n")
@@ -264,7 +264,7 @@ func (g *Generator) parsePackage(directory string, names []string, text interfac
 // check type-checks the package. The package must be OK to proceed.
 func (pkg *Package) check(fs *token.FileSet, astFiles []*ast.File) {
 	pkg.defs = make(map[*ast.Ident]types.Object)
-	config := types.Config{FakeImportC: true}
+	config := types.Config{Importer: importer.Default(), FakeImportC: true}
 	info := &types.Info{
 		Defs: pkg.defs,
 	}