maintenance-depends and a new tool selector for it

[geekosaur]

Describe the feature request
The idea is to support things that are per-project or per-package, but aren't strictly build tools: formatters, changelog-d, etc. This would also provide a more principled approach to cabal exec ghc (see #10196).

Additional context
Other build tools, such as npm, already support the concept of dev tools independent of build tools. But the primary reason for this is for code formatters, which in the Haskell ecosystem require you to pin formatter versions. There are currently few ways to deal with multiple projects that require different versions of e.g. fourmolu without resorting to external facilities (nix-shell, direnv, etc.).

This proposal involves two changes:

1. maintenance-depends for tools that are part of development but not of building/testing/etc.
2. A new selector (currently spitballing tool but I'm not wedded to it) to select these, ghc (replacing the cabal exec ghc hack), and possibly build-tool-depends.

[geekosaur]

There are a number of things that need to be ironed out here, the most important of which is that in many cases you may want tools to be per project instead of per package: consider our own use of changelog-d and fourmolu. But I think selectors are by package, so that may be the wrong approach for maintenance tools if we want those in cabal.project instead of package cabal files.

[ulysses4ever]

A silly question to those more knowledgeable than I: is it impossible to add this new field to the project file format and teach cabal to cabal run them anyway, without touching the package format?

[geekosaur]

I think project-level information doesn't support dependencies; cabal considers that package level. It is possible that doing this with cabal as it currently exists would require a utils-type package.

[pranaysashank]

Related issues others have opened over the years: #9230, #10016

[ulysses4ever]

Should we close this as a dup of #9230? The description here is valuable but at least it's linked from #9230 now...