Fix #6915 config set recreates global-project directory, when required

[mpilgrem]

See:

• stack config set fails if the global-project directory is missing/incomplete #6915

• Any changes that could be relevant to users have been recorded in ChangeLog.md.

• The documentation has been updated, if necessary

Please also shortly describe how you tested your change. Bonus points for added tests!

[mpilgrem]

Currently, the recreation of the global-project directory happens in the first part of Stack.Config.withBuildConfig. The final part does other things, which are not of interest to Stack's config set command.

The first part also outputs a user message, if one is specified in the project-level configuration file. I think Stack's config set command would want to skip that.

This pull request:

• separates the first part of withBuildConfig as:

  withConfigExtra ::
       forall a env. (HasConfig env, HasTerm env)
    => Bool
       -- ^ Report user message in the project-level configuration file, if
       -- a user message is present?
    -> (ConfigExtra -> RIO env a)
    -> RIO env a

• introduces the new type ConfigExtra for the information provided by the first part of withBuildConfig used by the remaining part;

• makes use of withConfigExtra in Stack.ConfigCmd.cfgCmdSet;

• adds an integration test for the bug fix 6915-config-set-outside-project.