platforms: remove errdefs dependency

This cleans up the platforms package from dependencies that are not strictly
needed. This is in preparation of making this package a separate module, which
can be shared by plugins, and containerd versions (as well as external consumers),

- Remove dependency on the errdefs package: most uses of these error
  definitions were used internally, and other errors may not be useful
  for external consumers as sentinel errors.
- ErrInvalidArgument may be a potential exception, although a look at
  current uses of this package shows that there's no special handling of
  invalid parameters vs other errors (all would boil down to "the passed
  platform is invalid" (either the format, or parsing is not implemented
  on a specific platform)
- Remove uses of the convenience "Platform" alias in favor of using the
  upstream (from OCI spec). Consumers of this package can still use the
  convenience alias, but make sure that function signatures do not imply
  that it's a different type (which can cause confusion).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

platforms/cpuinfo_linux.go

@@ -19,12 +19,12 @@ package platforms
 import (
 	"bufio"
 	"bytes"
+	"errors"
 	"fmt"
 	"os"
 	"runtime"
 	"strings"
 
-	"github.com/containerd/containerd/errdefs"
 	"golang.org/x/sys/unix"
 )
 
@@ -70,7 +70,7 @@ func getCPUInfo(pattern string) (info string, err error) {
 		return "", err
 	}
 
-	return "", fmt.Errorf("getCPUInfo for pattern %s: %w", pattern, errdefs.ErrNotFound)
+	return "", fmt.Errorf("getCPUInfo for pattern %s: %w", pattern, errNotFound)
 }
 
 // getCPUVariantFromArch get CPU variant from arch through a system call
@@ -83,7 +83,7 @@ func getCPUVariantFromArch(arch string) (string, error) {
 	if arch == "aarch64" {
 		variant = "8"
 	} else if arch[0:4] == "armv" && len(arch) >= 5 {
-		//Valid arch format is in form of armvXx
+		// Valid arch format is in form of armvXx
 		switch arch[3:5] {
 		case "v8":
 			variant = "8"
@@ -101,7 +101,7 @@ func getCPUVariantFromArch(arch string) (string, error) {
 			variant = "unknown"
 		}
 	} else {
-		return "", fmt.Errorf("getCPUVariantFromArch invalid arch: %s, %w", arch, errdefs.ErrInvalidArgument)
+		return "", fmt.Errorf("getCPUVariantFromArch invalid arch: %s, %w", arch, errInvalidArgument)
 	}
 	return variant, nil
 }
@@ -112,11 +112,10 @@ func getCPUVariantFromArch(arch string) (string, error) {
 // This is to cover running ARM in emulated environment on x86 host as this field in /proc/cpuinfo
 // was not present.
 func getCPUVariant() (string, error) {
-
 	variant, err := getCPUInfo("Cpu architecture")
 	if err != nil {
-		if errdefs.IsNotFound(err) {
-			//Let's try getting CPU variant from machine architecture
+		if errors.Is(err, errNotFound) {
+			// Let's try getting CPU variant from machine architecture
 			arch, err := getMachineArch()
 			if err != nil {
 				return "", fmt.Errorf("failure getting machine architecture: %v", err)