Include build-tools to dependencies

[the-dr-lazy]

Snack doesn't include build-tools specified in package.yaml into the build environment.
As an example:

library:
  source-dirs: src
  ghc-options:
    - -F
    - -pgmF=record-dot-preprocessor
  dependencies:
    - record-hasfield
  build-tools:
    - record-dot-preprocessor

With the above package.yaml file Snack will fail to build and throws the following error:

ghc: could not execute record-dot-preprocessor

After adding record-dot-preprocessor to the dependencies Snack can build the project. So it would be rational to suppose build-tools as dependencies that should be provided by Snack.