Add support for "setup" components to target selector & new-build

Currently, we cannot refer to `custom-setup` components

See [`Distribution.Client.TargetSelector`](https://github.com/haskell/cabal/blob/master/cabal-install/Distribution/Client/TargetSelector.hs) for currently supported syntax.

In order to reify custom-setup components in the target selector syntax, we'd have to introduce a new (singleton) namespace "`setup`", which has ~~only~~at most a single inhabitant (hence a singleton -- however, this inhabitant only exists for `build-type:custom` packages), also named "`setup`". Consequently, this gives us the following ways to refer to the custom setup script for a package foo

- `foo:setup` (package-name : component-name)
- `setup` (component-name)
- `setup:setup` (namespace-name : component-name)
- `foo:setup:setup` (package-name : namespace-name : component-name )
- `:pkg:foo:setup:setup` (explicitly qualified package-name : namespace-name : component-name )

(Optional) Moreover, we could add the "filter" `setups`, which then gives us the ability to refer to custom-setups components of multiple packages via

- `setups`
- `all:setups`
- `:all:setups`
- `foo:setups` (trivial: all `setup` components in the package `foo`; IOW, same as `foo:setup:setup`)

This may be a pre-requisite for #4293 which needs the ability to `cabal new-build setup(s)` w/o building all remaining targets.