Add external test library modules to import ordering in style doc

[Jimbo4350]

No description provided.

[carbolymer]

shouldn't the order be reversed similarly to non-test modules i.e.:

4. External testing library modules.
5. Local test modules.

[newhoggy]

The downside to having import groups like this in the style guide is that what ends up happening is PRs get block in reviews purely for import grouping reasons or otherwise get missed by the coder and reviewer and get merged anyway. One affects our velocity as PRs that are otherwise ready don't get merged and we end up doing more rebases and CI jobs whilst the other means more of our code doesn't match the style guide.

I have a counter-proposal:

• Delete the section on import group
• Add a section that our code must pass stylish-haskell checks if there isn't already one
• Add import grouping rules to our stylish-haskell config to enforce the style we want We write [import grouping rules]
• Any import grouping rules we cannot express in the stylish-haskell configuration are discarded.
• Make stylish-haskell a required check

This has the following advantages:

• All future merges to the master are guaranteed to follow our import grouping rules.
• Contributors get much faster feedback and don't have to wait for a review to realise their import grouping fails the style guide
• Reviewers have one less thing to worry about making reviews much faster.
• PRs get merged faster and suffer fewer rebases and conflicts
• If stylish-haskell is enabled locally, the local file will automatically match the style guide relieving the developer of one more thing to think about

[Jimbo4350]

Closing this PR as I agree with John: #6 (comment)