Allows imports of the NPM package added by 1a093ef482
to be replaced with import("http://localhost:9090/pkg/pkg.js"), so that
changes can be made in parallel to both the module and code that uses
it (without any need for NPM publishing or even building of the package).
Updates #5415
Signed-off-by: Mihai Parparita <mihai@tailscale.com>
