Avoid importing components from /src directories of any modules

[gabriele-ct]

We found out that imports like
import module from '@commercetools-docs/somepackage/src/somefile';

can lead to issues due to the fact that src directory is not always available in prod build.

I'd reccomend:

• Extend the eslint rule added by @ColinRosati and make it valid for all the packages
• Ensure that each package exports the public components directly in the root index file
• Remove any import (both in docs-kit and in docs) that taps into the src directory

Optional (but I think it's important):

• Change each package's package.json file ensuring that the src folder is not included in the npm package