remove uses of github.com/docker/docker/builder/remotecontext package

This package is part of the classic builder, and was currently only used
for the IsURL utility, which is a very rudimentary check for a string having
a "https://" or "http://" scheme.

This patch copies the code as non-exported functions where they're used to
remove the dependency.

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

build/utils.go

@@ -11,7 +11,6 @@ import (
 
 	"github.com/docker/buildx/driver"
 	"github.com/docker/cli/opts"
-	"github.com/docker/docker/builder/remotecontext/urlutil"
 	"github.com/moby/buildkit/util/gitutil"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -26,8 +25,15 @@ const (
 	mobyHostGatewayName = "host-gateway"
 )
 
+// isHTTPURL returns true if the provided str is an HTTP(S) URL by checking if it
+// has a http:// or https:// scheme. No validation is performed to verify if the
+// URL is well-formed.
+func isHTTPURL(str string) bool {
+	return strings.HasPrefix(str, "https://") || strings.HasPrefix(str, "http://")
+}
+
 func IsRemoteURL(c string) bool {
-	if urlutil.IsURL(c) {
+	if isHTTPURL(c) {
 		return true
 	}
 	if _, err := gitutil.ParseGitRef(c); err == nil {