debug: Launch Package template triggered with no config does not match the templates in `package.json`

[hyangah]

In #3387, the "Launch Package" template in package.json was updated to use ${workspaceFolder}. But we missed to update the template in https://github.com/golang/vscode-go/blob/2b8494a8d05fea085d9f2ce3e6d0f856528916a5/extension/src/goDebugConfiguration.ts#L63-L69 that is referenced when launching debugging without launch.json for the first time. I think the right fix is to make the goDebugConfiguration read the template in package.json instead of maintaining the copy in the code, so we update only one place in the future.