Require cabal.project packages and/or optional-packages field

[tbidne]

Resolves #7401.

Turns the prior warning into an error. This has the effect that the misleading messages are never logged, since the new error causes an immediate failure.

I attempted to add as many test cases as I could think of, so there may be some redundancy (e.g. I checked explicit vs. implicit targets, and evidently it doesn't matter).

I also added the project file path to the error message. This is grabbed here:

establishProjectBaseContextWithRoot
  :: Verbosity
  -> ProjectConfig
  -> ProjectRoot
  -> CurrentCommand
  -> IO ProjectBaseContext
establishProjectBaseContextWithRoot verbosity cliConfig projectRoot currentCommand = do
  let haddockOutputDir = flagToMaybe (packageConfigHaddockOutputDir (projectConfigLocalPackages cliConfig))
  let distDirLayout = defaultDistDirLayout projectRoot mdistDirectory haddockOutputDir

  ...

  -- Get path from distDirLayout here...
  let projPath = distProjectFileMain (distProjectFile distDirLayout)
  when (null (projectPackages projectConfig) && null (projectPackagesOptional projectConfig)) $
    dieWithException verbosity (ProjectConfigNoPackages projPath)

This seems right to me, though someone should double-check this makes sense.

Thanks!

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