--docs flag does not work on elm/* packages with cross-package kernel imports

[harrysarson]

• This is a regression report from elm 0.19.0 to 0.19.1.

• When I run elm-0.19.0 make --docs=documentation.json in a kernel package it successfully generates the docs and exits with 0 code.

• When I run elm-0.19.1 make --docs=documentation.json in the same package it produces the following error and exits with code 1.

  Detected problems in 1 module.
  -- MODULE NOT FOUND -------------------------------------- src/Test/Internal.elm
  
  You are trying to import a `Elm.Kernel.Debug` module:
  
  3| import Elm.Kernel.Debug
            ^^^^^^^^^^^^^^^^
  I checked the "dependencies" and "source-directories" listed in your elm.json,
  but I cannot find it! Maybe it is a typo for one of these names?
  
      Elm.Kernel.Test
      Elm.Kernel.HtmlAsJson
      Html.Keyed
      Json.Encode
  
  Hint: If it is not a typo, check the "dependencies" and "source-directories" of
  your elm.json to make sure all the packages you need are listed there!
  

---

To reproduce:

1. git clone https://github.com/elm-explorations/test
2. cd test
3. elm-0.19.0 make --docs=documentation.json
4. elm-0.19.1 make --docs=documentation.json

---

Related https://github.com/elm-explorations/test/pull/145#issuecomment-652281979

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