go/packages: remove usage of golang.org/x/xerrors

As of golang/go#50827, gopls no longer needs to build at Go 1.12. I
believe this was the only reason to continue using xerrors in
go/packages.

Remove this usage as a step toward eliminating the xerrors dependency
from x/tools.

For golang/go#52442

Change-Id: I8f7396491be58606be6a648ebbf2180bdbc907b5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/401154
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

go/packages/golist.go

@@ -26,7 +26,6 @@ import (
 	"golang.org/x/tools/go/internal/packagesdriver"
 	"golang.org/x/tools/internal/gocommand"
 	"golang.org/x/tools/internal/packagesinternal"
-	"golang.org/x/xerrors"
 )
 
 // debug controls verbose logging.
@@ -953,7 +952,7 @@ func (state *golistState) invokeGo(verb string, args ...string) (*bytes.Buffer,
 		if !ok {
 			// Catastrophic error:
 			// - context cancellation
-			return nil, xerrors.Errorf("couldn't run 'go': %w", err)
+			return nil, fmt.Errorf("couldn't run 'go': %w", err)
 		}
 
 		// Old go version?

go/packages/packagestest/export.go

@@ -81,7 +81,6 @@ import (
 	"golang.org/x/tools/go/packages"
 	"golang.org/x/tools/internal/span"
 	"golang.org/x/tools/internal/testenv"
-	"golang.org/x/xerrors"
 )
 
 var (
@@ -247,7 +246,7 @@ func Export(t testing.TB, exporter Exporter, modules []Module) *Exported {
 			switch value := value.(type) {
 			case Writer:
 				if err := value(fullpath); err != nil {
-					if xerrors.Is(err, ErrUnsupported) {
+					if errors.Is(err, ErrUnsupported) {
 						t.Skip(err)
 					}
 					t.Fatal(err)
@@ -339,7 +338,7 @@ func Symlink(source string) Writer {
 			mode := os.ModePerm
 			if err == nil {
 				mode = stat.Mode()
-			} else if !xerrors.Is(err, os.ErrNotExist) {
+			} else if !errors.Is(err, os.ErrNotExist) {
 				// We couldn't open the source, but it might exist. We don't expect to be
 				// able to portably create a symlink to a file we can't see.
 				return symlinkErr