Replace pkg-config with pkgconf when the distribution is brew (#26891)

* Homebrew has deprecated pkg-config. `brew install pkg-config` now
installs pkgconf. Accordingly, this PR adjusts the expectations
of `conf-pkg-config` to depend on `pkgconf`.

* conf-pkg-config: let's leave the old ones untouched

Signed-off-by: Marcello Seri <[email protected]>

---------

Signed-off-by: Marcello Seri <[email protected]>
Co-authored-by: Emma Jin <[email protected]>

Author: mseri

packages/conf-pkg-config/conf-pkg-config.3/opam

@@ -22,7 +22,7 @@ depexts: [
   ["pkgconfig"] {os-distribution = "ol" & os-version <= "7"}
   ["pkgconf"] {os-distribution = "alpine"}
   ["pkg-config"] {os-distribution = "nixos"}
-  ["pkg-config"] {os = "macos" & os-distribution = "homebrew"}
+  ["pkgconf"] {os = "macos" & os-distribution = "homebrew"}
   ["pkgconfig"] {os = "macos" & os-distribution = "macports"}
   ["pkgconf"] {os = "freebsd"}
   ["pkgconf-pkg-config"] {os-distribution = "rhel" & os-version >= "8"}