Fix -P with passing ko:// through. (#163)

Turns out I broke this.  Thanks to @bobcatfish for reporting the issue.

With this change:
```
ko resolve -Pf cmd/ko/test/test.yaml > /dev/null
2020/05/04 08:44:35 Using base gcr.io/distroless/static:nonroot for github.com/google/ko/cmd/ko/test
2020/05/04 08:44:36 Building github.com/google/ko/cmd/ko/test
2020/05/04 08:44:37 Publishing gcr.io/mattmoor-knative/github.com/google/ko/cmd/ko/test:latest
2020/05/04 08:44:39 Published gcr.io/mattmoor-knative/github.com/google/ko/cmd/ko/test@sha256:ee655510172b429dbce619fc69677621d71cb824cbbf2a21746d700127257ec4
```

I can cut v0.5.1 once this lands.

pkg/publish/default.go

@@ -24,6 +24,7 @@ import (
 	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/remote"
+	"github.com/google/ko/pkg/build"
 )
 
 // defalt is intentionally misspelled to avoid keyword collision (and drive Jon nuts).
@@ -95,6 +96,7 @@ func NewDefault(base string, options ...Option) (Interface, error) {
 
 // Publish implements publish.Interface
 func (d *defalt) Publish(img v1.Image, s string) (name.Reference, error) {
+	s = strings.TrimPrefix(s, build.StrictScheme)
 	// https://github.com/google/go-containerregistry/issues/212
 	s = strings.ToLower(s)
 

pkg/publish/default_test.go

@@ -26,6 +26,7 @@ import (
 
 	"github.com/google/go-containerregistry/pkg/name"
 	"github.com/google/go-containerregistry/pkg/v1/random"
+	"github.com/google/ko/pkg/build"
 )
 
 func TestDefault(t *testing.T) {
@@ -77,7 +78,7 @@ func TestDefault(t *testing.T) {
 	if err != nil {
 		t.Errorf("NewDefault() = %v", err)
 	}
-	if d, err := def.Publish(img, importpath); err != nil {
+	if d, err := def.Publish(img, build.StrictScheme+importpath); err != nil {
 		t.Errorf("Publish() = %v", err)
 	} else if !strings.HasPrefix(d.String(), tag.Repository.String()) {
 		t.Errorf("Publish() = %v, wanted prefix %v", d, tag.Repository)
@@ -141,7 +142,7 @@ func TestDefaultWithCustomNamer(t *testing.T) {
 	if err != nil {
 		t.Errorf("NewDefault() = %v", err)
 	}
-	if d, err := def.Publish(img, importpath); err != nil {
+	if d, err := def.Publish(img, build.StrictScheme+importpath); err != nil {
 		t.Errorf("Publish() = %v", err)
 	} else if !strings.HasPrefix(d.String(), repoName) {
 		t.Errorf("Publish() = %v, wanted prefix %v", d, tag.Repository)
@@ -205,7 +206,7 @@ func TestDefaultWithTags(t *testing.T) {
 	if err != nil {
 		t.Errorf("NewDefault() = %v", err)
 	}
-	if d, err := def.Publish(img, importpath); err != nil {
+	if d, err := def.Publish(img, build.StrictScheme+importpath); err != nil {
 		t.Errorf("Publish() = %v", err)
 	} else if !strings.HasPrefix(d.String(), repoName) {
 		t.Errorf("Publish() = %v, wanted prefix %v", d, tag.Repository)
@@ -278,7 +279,7 @@ func TestDefaultWithReleaseTag(t *testing.T) {
 	if err != nil {
 		t.Errorf("NewDefault() = %v", err)
 	}
-	if d, err := def.Publish(img, importpath); err != nil {
+	if d, err := def.Publish(img, build.StrictScheme+importpath); err != nil {
 		t.Errorf("Publish() = %v", err)
 	} else if !strings.HasPrefix(d.String(), repoName) {
 		t.Errorf("Publish() = %v, wanted prefix %v", d, tag.Repository)