`maintenance-depends` and a new `tool` selector for it

**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`.