Adapt OPAL_LOAD_PATH env variable to webpack standards

[wied03]

Lower priority.

Rather than have a separate OPAL_LOAD_PATH setting, might want to use resolve.moduleDirectories in the webpack config instead. This is changing with webpack 2.0 though, so probably best to wait until that happens.

When this is done, look into removing absolute paths from the webpack requires. Those make it into the cache and we don't want to make the cache dirty just because directories are moved around.

[wied03]

In Webpack 2.0, resolve.root, resolve.moduleDirectories, and resolve.fallback => resolve.modules.

[wied03]

One issue with implementing this is that loaders really shouldn't modify the webpack config. That should be a plugin's job.