Allow 'nested' aliases in resolve config

[nosnickid]

In my project, I have nested my private aliases in a namespace-ish manner.

My resolve.alias looks somewhat like:

resolve: {
  alias: {
    'ns/module1': path.resolve('...'),
    'ns/module2': path.resolve('...')
  }
}

This functions in webpack but wasn't functioning with the slightly simpler implementation of matchAlias in the webpack resolver.

This patch looks for a common prefix in the source and alias in matchAlias and fixes configs as above.

[benmosher]

Ah, interesting. I had not considered this given that it was not in the Webpack table.

I wonder if path.relative could be used for this... something like a path version of indexOf that is separator-agnostic...

Thanks for this! and thanks for the tests!