Add a doctest-% pattern target

[philderbeast]

Fixes #11798. Adds a make help item doctest-PACKAGENAME and targets for running doctests for a single package using a doctest-% pattern rule.

$ make help
...
doctest-PACKAGENAME            Run doctests for a single package (replace PACKAGENAME).
doctest-install                Install doctest tool needed for running doctests.
doctest                        Run doctests in all packages.

$ make doctest-PACKAGENAME
Please use one of the following targets:
doctest-Cabal
doctest-Cabal-described
doctest-Cabal-syntax
doctest-cabal-install
doctest-cabal-install-solver
doctest-cabal-testsuite

Note

There were doctests in cabal-testsuite. These weren't being run before and cannot be run with cd cabal-testsuite && cabal doctest so these use REPL_WITH_DOCTEST. I renamed DOCTEST to CABAL_DOCTEST.

---

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[ulysses4ever]

thanks for the split but the PR title is still far from what is actually done because of how much is done. I think adding convenience targets and switching between cabal doctest and cabal repl should be different PRs. The latter is highly questionable for me while the former is a no brainer (well, if not for the metaprogramming).