It isn't possible to develop mynewt using conventional go methods, or to use it as a dependency in another package.
The problem is that the https://mynewt.apache.org/newt/ doesn't return valid results for subdirectories.
The first above command returns valid redirects for github, but the second just returns a 404. This causes the git fetch to fail.
If something such as mcuboot/imgtool tries to use https://mynewt.apache.org/newt/newt/image as a dependency, it also fails to fetch this.