- Disable cabal-install's haddock on CI
- Temporarily disable cabal check (`-main-is` in tests)
- Remove this is not a lib note
- Tell to run git update-index --assume-unchanged

    ... when changing to -dev or -monolithic configuration.

    Run --no-assume-unchanged unconditionally, when
    changing to the production build.
- Improve auto-gen note in cabal-install.cabal
- Try to build haddock (with GHC-8.4.3 only)

This script contains similar commands to appveyor.yml, except it relies
on cabal new-build, so should be safe to run locally.

Using this script can ease Travis stress.

ATM I see following tests failing:

- PackageTests/Backpack/Includes3/setup-external-ok.test.hs
    - https://github.com/haskell/cabal/issues/5039
- PackageTests/CustomPlain/setup.test.hs
    - `<command line>: cannot satisfy -package-id Cabal-2.1.0.0-inplace`
- PackageTests/CustomDep/sandbox.test.hs
    - `.../dist-sandbox-3402c05b/setup/setup: createProcess: runInteractiveProcess: exec: permission denied (Permission denied)`