Polyfill `devDependencies` should be resolved from `polyfills/*`

[mhassan1]

In the case where a polyfill has devDependencies, we currently resolve those devDependencies from the root, rather than from the correct polyfills/*. In the case where there are multiple versions of those devDependencies, it could mean that we are resolving the wrong versions.

To reproduce:

• Add "whatwg-url": "14.0.0" to a package.json file in the URL polyfill
• Add the following to config.toml in the URL polyfill:

  [install]
  module = "whatwg-url"
  paths = [ "index.js" ]
  clean = [ "polyfill.js" ]

• See Error: Cannot find module 'whatwg-url/index.js' because whatwg-url@5 (which is in the root node_modules) does not have index.js (even though whatwg-url@14 does)

[mhassan1]

I think I see the problem. PR coming up.