Fix a couple of path separator/delimiter issues.

Note `lastIndexOf( , 0) === 0` idiom is effectively `startsWith()`.
For PACKAGE_DIRS Windows can't use `:` so use the portable `path.delimiter` instead.

For the `Npm.require()` part of the change, splitting by `path.sep` is an incorrect use as the parameter uses forward slashes.
The Node [docs](http://nodejs.org/api/modules.html#modules_modules) mention forward slashes specifically and realistically you'd have to use forward slash for the same code to work on Linux/Mac etc.

So split explicitly by forward slash.

The rest of the Meteor code base does a pretty good job of using `path.sep` and `path.join()` so these are essentially the only changes.