I have the following directory structure:
limejs/
closure/
closure/
goog/
base.js
projects/
myGame/
myGame.html
server.js
<
You're probably using express.static(dir). Files are only served from dir down, everything else is invisible. Same goes for any other static file server (Apache, ningx, etc).
You need to either serve files from higher in the directory tree, so that closure/closure/goog/base.js is accessible (from limejs in this case), or copy/link the files into your game folder.
If you're on linux/mac, ln -s closure/closure projects/myGame/closure should do the trick, then you can include <script src="closure/goog/base.js">.