Update repo-detection code for jupyterlab_widgets

[jasongrout]

We have some code (that actually doesn't work, I think?) at

ipywidgets/jupyterlab_widgets/setup.py

Lines 49 to 57 in 5532c91

	# if the static assets already exist, do not invoke npm so we can make a wheel
	# from the sdist package, since the npm build really only works from this
	# repo.
	jsbuild = []
	if all(os.path.exists(f) for f in jstargets):
	jsbuild.append(install_npm(HERE, build_cmd="build:prod", npm=["jlpm"]))
	jsbuild.append(ensure_targets(jstargets))
	cmdclass["jsdeps"] = combine_commands(*jsbuild)

(and soon in master after #3086) to detect if we are building jupyterlab_widgets in the repo, to avoid recompiling js files many times.

The updated jlab extension cookiecutter has better code at

https://github.com/jupyterlab/extension-cookiecutter-ts/blob/b13df389efd10fbccba441537aa198e64f623fa4/%7B%7Bcookiecutter.python_name%7D%7D/setup.py#L53-L57

I think we should switch our code to using code like is currently in the jlab extension cookiecutter.