identifiers: use common package for identifier validation

A few days ago, we added validation for namespaces. We've decided to
expand these naming rules to include containers. To facilitate this, a
common package `identifiers` now provides a common validation area.
These rules will be extended to apply to task identifiers, snapshot keys
and other areas where user-provided identifiers may be used.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

namespaces/validate_test.go

@@ -1,79 +0,0 @@
-package namespaces
-
-import (
-	"testing"
-
-	"github.com/pkg/errors"
-)
-
-func TestValidNamespaces(t *testing.T) {
-	for _, testcase := range []struct {
-		input string
-		err   error
-	}{
-		{
-			input: "default",
-		},
-		{
-			input: "default-default",
-		},
-		{
-			input: "default--default",
-		},
-		{
-			input: "containerd.io",
-		},
-		{
-			input: "foo.boo",
-		},
-		{
-			input: "swarmkit.docker.io",
-		},
-		{
-			input: "zn--e9.org", // or something like it!
-		},
-		{
-			input: ".foo..foo",
-			err:   errNamespaceInvalid,
-		},
-		{
-			input: "foo/foo",
-			err:   errNamespaceInvalid,
-		},
-		{
-			input: "foo/..",
-			err:   errNamespaceInvalid,
-		},
-		{
-			input: "foo..foo",
-			err:   errNamespaceInvalid,
-		},
-		{
-			input: "foo.-boo",
-			err:   errNamespaceInvalid,
-		},
-		{
-			input: "-foo.boo",
-			err:   errNamespaceInvalid,
-		},
-		{
-			input: "foo.boo-",
-			err:   errNamespaceInvalid,
-		},
-		{
-			input: "foo_foo.boo_underscores", // boo-urns?
-			err:   errNamespaceInvalid,
-		},
-	} {
-		t.Run(testcase.input, func(t *testing.T) {
-			if err := Validate(testcase.input); errors.Cause(err) != testcase.err {
-				t.Log(errors.Cause(err), testcase.err)
-				if testcase.err == nil {
-					t.Fatalf("unexpected error: %v != nil", err)
-				} else {
-					t.Fatalf("expected error %v to be %v", err, testcase.err)
-				}
-			}
-		})
-	}
-}