fix incorrect import path in copy + preserveModules mode

[lgodziejewski]

Description

When using copy + preserveModules, the final import path was incorrect:

e.g. dir structure:

src
  components
    ComponentA
      ComponentA.js
      assets
        fancy-asset.svg

so the original import was ./assets/fancy-asset.svg

after building, the structure was:

build
  components
    ComponentA
      ComponentA.js
  assets
    fancy-asset.svg

but the import path was ../../../assets/fancy-assets.svg instead of ../../assets/fancy-assets.svg.

The reason was that the final path was calculated based on the "wrapper file" (i.e. src/components/ComponentA/assets/fanc-asset.svg.js) instead of the importing file - I changed the implementation and the existing test.

Let me know if that's fine or perhaps if I should leave the existing case and just add a new one.

[Harris-Miller]

Bump