extra-pkg-config-dirs directive

It would be helpful to have an `extra-pkg-config-dirs` directive, similar to the current `extra-lib-dirs` and `extra-include-dirs` directives. As a NixOS user, I have to include both of these in my `cabal.project` but still need to set `PKG_CONFIG_PATH` manually in my environment.

Start by locating the existing handling for the `extra-lib-dirs` and `extra-include-dirs` directives in Cabal's project configuration. Trace how those directories affect package configuration, then verify that an `extra-pkg-config-dirs` directive would set the corresponding lookup path without requiring a manual `PKG_CONFIG_PATH`; add or update focused tests for the project configuration behavior.

Written by the indexing model from the issue text.