builder/remotecontext: use net/url instead of urlutil

urlutil.IsUrl() was merely checking if the url had a http(s)://
prefix, which is just as well handled through using url.Parse()

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

cli/command/image/build/internal/git/gitutils_test.go

@@ -24,6 +24,14 @@ func TestParseRemoteURL(t *testing.T) {
 		url      string
 		expected gitRepo
 	}{
+		{
+			doc: "git scheme uppercase, no url-fragment",
+			url: "GIT://github.com/user/repo.git",
+			expected: gitRepo{
+				remote: "git://github.com/user/repo.git",
+				ref:    "master",
+			},
+		},
 		{
 			doc: "git scheme, no url-fragment",
 			url: "git://github.com/user/repo.git",