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

vscode-go/extension/src/goDebugConfiguration.ts

Lines 63 to 69 in 2b8494a

	config: {
	name: 'Launch Package',
	type: this.defaultDebugAdapterType,
	request: 'launch',
	mode: 'auto',
	program: '${fileDirname}'
	}

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.