Use require() when loading NPM tasks

I wrote a library using grunt to build itself. Let's call it L.
I wrote a project using grunt to build itself, and using L as a dependency. Let's call it P.
Both L and P are using the same grunt plugin. Let's call it G.

Because npm does install a library just once in a node_modules tree, here is the hierarchy of the P project after running `npm install`:

```
./P
./P/node_modules
./P/node_modules/G
./P/node_modules/L
```

But there is no `./P/node_modules/L/node_modules/G` and that makes grunt fail to build L.

Why is grunt searching for a `./node_modules//package.json` file instead of just requiring it like any other node module?

Start by tracing Grunt's NPM task-loading path and reproduce the P/L/G dependency layout described in the issue. Determine how the loader currently searches for G and verify that the completed change lets L load the shared plugin from P's node_modules tree, with a regression test for this layout.

Written by the indexing model from the issue text.