code-organization

synopsis: Code organization
prs: #6599 #6642 #6734 #6745 #6746 #6817 #6818 #6821 #6867

description: {

- Split `Distribution.Client.Types` module
- Move cabal-testsuite code into `src/`
- Add `ProjectFlags`
- Add `NixStyleOptions`
- Internal refactorings to install command.
- Make own modules for `InstallMethod` and `OverwritePolicy`
- Make cabal-install compilable with `NoImplicitPrelude`
- Refactor shared `TargetProblem` data types into their own module.
- Template `cabal-install.cabal` using zinza

}