cmd/go/internal/modfetch: use errors.ErrUnsupported

CL 473935 added errors.ErrUnsupported, let's use it.

Updates #41198

Change-Id: If6534d19cb31ca979ff00d529bd6bdfc964a616d
Reviewed-on: https://go-review.googlesource.com/c/go/+/476135
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/modfetch/codehost/codehost.go

@@ -201,19 +201,6 @@ func (noCommitsError) Is(err error) bool {
 	return err == fs.ErrNotExist
 }
 
-// ErrUnsupported indicates that a requested operation cannot be performed,
-// because it is unsupported. This error indicates that there is no alternative
-// way to perform the operation.
-//
-// TODO(#41198): Remove this declaration and use errors.ErrUnsupported instead.
-var ErrUnsupported = unsupportedOperationError{}
-
-type unsupportedOperationError struct{}
-
-func (unsupportedOperationError) Error() string {
-	return "unsupported operation"
-}
-
 // AllHex reports whether the revision rev is entirely lower-case hexadecimal digits.
 func AllHex(rev string) bool {
 	for i := 0; i < len(rev); i++ {

src/cmd/go/internal/modfetch/codehost/vcs.go

@@ -288,7 +288,7 @@ func (r *vcsRepo) loadBranches() {
 }
 
 func (r *vcsRepo) CheckReuse(old *Origin, subdir string) error {
-	return fmt.Errorf("vcs %s: CheckReuse: %w", r.cmd.vcs, ErrUnsupported)
+	return fmt.Errorf("vcs %s: CheckReuse: %w", r.cmd.vcs, errors.ErrUnsupported)
 }
 
 func (r *vcsRepo) Tags(prefix string) (*Tags, error) {
@@ -412,7 +412,7 @@ func (r *vcsRepo) RecentTag(rev, prefix string, allowed func(string) bool) (tag
 	}
 	defer unlock()
 
-	return "", vcsErrorf("vcs %s: RecentTag: %w", r.cmd.vcs, ErrUnsupported)
+	return "", vcsErrorf("vcs %s: RecentTag: %w", r.cmd.vcs, errors.ErrUnsupported)
 }
 
 func (r *vcsRepo) DescendsFrom(rev, tag string) (bool, error) {
@@ -422,12 +422,12 @@ func (r *vcsRepo) DescendsFrom(rev, tag string) (bool, error) {
 	}
 	defer unlock()
 
-	return false, vcsErrorf("vcs %s: DescendsFrom: %w", r.cmd.vcs, ErrUnsupported)
+	return false, vcsErrorf("vcs %s: DescendsFrom: %w", r.cmd.vcs, errors.ErrUnsupported)
 }
 
 func (r *vcsRepo) ReadZip(rev, subdir string, maxSize int64) (zip io.ReadCloser, err error) {
 	if r.cmd.readZip == nil && r.cmd.doReadZip == nil {
-		return nil, vcsErrorf("vcs %s: ReadZip: %w", r.cmd.vcs, ErrUnsupported)
+		return nil, vcsErrorf("vcs %s: ReadZip: %w", r.cmd.vcs, errors.ErrUnsupported)
 	}
 
 	unlock, err := r.mu.Lock()

src/cmd/go/internal/modfetch/coderepo.go

@@ -608,7 +608,7 @@ func (r *codeRepo) convert(info *codehost.RevInfo, statVers string) (*RevInfo, e
 	}
 	if pseudoBase == "" {
 		tag, err := r.code.RecentTag(info.Name, tagPrefix, tagAllowed)
-		if err != nil && !errors.Is(err, codehost.ErrUnsupported) {
+		if err != nil && !errors.Is(err, errors.ErrUnsupported) {
 			return nil, err
 		}
 		if tag != "" {