cmd/stringer: revert CL 40403

Revert "cmd/stringer: use source importer when available"

This reverts CL 40403.

The idea is to avoid type-checking and use just parsing, which should be
enough for stringer.

Separately reopening golang/go#10249 because the original change closed that issue,
but the change is itself causing other problems as described in the discussion
at golang/go#25650.

This reversion restores the old behavior of stringer and will be followed
with other fixes if they can be worked out.

Change-Id: I8404d78da08043ede1a36b0e135a3fc7fdf6728d
Reviewed-on: https://go-review.googlesource.com/121884
Reviewed-by: Ian Lance Taylor <iant@golang.org>

endtoend_test.go

@@ -33,7 +33,7 @@ func TestEndToEnd(t *testing.T) {
 	defer os.RemoveAll(dir)
 	// Create stringer in temporary directory.
 	stringer := filepath.Join(dir, "stringer.exe")
-	err = run("go", "build", "-o", stringer)
+	err = run("go", "build", "-o", stringer, "stringer.go")
 	if err != nil {
 		t.Fatalf("building stringer: %s", err)
 	}

stringer.go

@@ -66,6 +66,7 @@ import (
 	"go/build"
 	exact "go/constant"
 	"go/format"
+	"go/importer"
 	"go/parser"
 	"go/token"
 	"go/types"
@@ -268,7 +269,7 @@ 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{Importer: defaultImporter(), FakeImportC: true}
+	config := types.Config{Importer: importer.Default(), FakeImportC: true}
 	info := &types.Info{
 		Defs: pkg.defs,
 	}