Import packages from project-cabal/pkgs

- Move constraints to project-cabal/constraints.config
- Remove duplicate package groups
- Groups coverage, doctest and validate are the same as the default group
- Remove duplicate validate-libonly package group
- Both validate and validate-libonly are the same
- Move Cabal & Cabal-syntax to pkgs/cabal.config
- Add install package group
- Add benchmarks package group
- Add tests package group
- Both default and libonly groups were the same set of packages that I renamed to tests
- Put install group before tests and benchmarks
- Don't repeat packages
- Add TODO and REVIEW comments on allow-newer exceptions
- Move latest.ghc configuration
- Put program-options first & separate imports
- Remove optional-packages
- Don't reiterate default value for tests and benchmarks
- Don't reiterate default value for optimization
- Add ghc-options.config
- Rename to ghc-latest.config
- Use -Werror in ghc-options.config
- Don't include ghc-options.config for doctest project
- Add project-config/pkgs.config
- Add project-config/pkgs.config importing all package groups
- Move Cabal-described to the cabal package group
- Remove cabal.project.buildinfo
- Have cabal.project.doctest import cabal.project
- Split integration tests into their own package group.
- Add back trailing newlines at EOF in projects
- Integration-tests.config needed for libonly
- Add a README for projects.