Fixes #20511 - Webpack config should look for nested deps

The current value of resolve.modules in config/webpack.config.js is
"path.join....'node_modules'"".
This value doesn't work when dependencies have been installed using
global style (npm install --global-style true).

In this style, which is what foreman-packaging uses, dependencies of
dependencies get nested like:

node_modules/dependencyA/node_modules/dependencyB

in a tree like structure. foreman-packaging needs to install in this
mode because the npm cache is stored
like a tree.

The solution I've been given in #webpack is to use a string
'node_modules' instead of path.join. The reason is that
webpack purposefully looks for nested dependencies in that case, but not
if you provide the full path.