Implement pkg-config --variables equivalent

CPS https://github.com/cps-org/cps/issues/4 is a relevant question here.

There are a few cases that I know of that need to be solved:
- To get the location of a binary tool with Qt you run `pkg-config QtCore --variable bindir`, then append the binary you want
- To get the location of a binary with wayland_scanner you run `pkg-config wayland-scanner --variable wayland_scanner`
- I see a number of pkgs looking at things like `libdir`, `bindir`, `libexedir`, as well as some custom directories like `pluginsdir`
- One X11 package uses this to distinguish whether it was built with thread-local support

Some of this is custom data, but some of this may well just want to act as an interface proxy in cps-config to point to existing data, the wayland_scanner example might just somehow reference the wayland_scanner component.

Start by reading the linked CPS issue and comparing the listed Qt, Wayland, X11, and directory-variable cases with the existing cps-config behavior. Clarify which variables are custom data and which should proxy existing CPS components or paths. Done means the supported equivalent cases are defined and implemented with verification for each listed use case.

Written by the indexing model from the issue text.