Remove deprecated things. (#162)

Fixes: https://github.com/google/ko/issues/158
Fixes: https://github.com/google/ko/issues/160

pkg/build/gobuild.go

@@ -168,40 +168,20 @@ func NewGo(options ...Option) (Interface, error) {
 	return gbo.Open()
 }
 
-const Deprecation158 = `NOTICE!
------------------------------------------------------------------
-We will start requiring ko:// in a coming release.  Please prefix
-the following import path for things to continue working:
-
-   %s
-
-For more information see:
-
-   https://github.com/google/ko/issues/158
-
------------------------------------------------------------------
-`
-
 // IsSupportedReference implements build.Interface
 //
 // Only valid importpaths that provide commands (i.e., are "package main") are
 // supported.
 func (g *gobuild) IsSupportedReference(s string) bool {
 	ref := newRef(s)
-	if p, err := g.importPackage(ref); err != nil {
-		if ref.IsStrict() {
-			log.Fatalf("%q is not supported: %v", ref.String(), err)
-		}
+	if !ref.IsStrict() {
 		return false
-	} else if p.IsCommand() {
-		if !ref.IsStrict() {
-			log.Printf(Deprecation158, s)
-		}
-		return true
-	} else if ref.IsStrict() {
-		log.Fatalf(`%q does not have "package main"`, ref.String())
 	}
-	return false
+	p, err := g.importPackage(ref)
+	if err != nil {
+		return false
+	}
+	return p.IsCommand()
 }
 
 // importPackage wraps go/build.Import to handle go modules.