treewide: pkgs.pkgconfig -> pkgs.pkg-config, move pkgconfig to alias.nix

continuation of #109595

pkgconfig was aliased in 2018, however, it remained in
all-packages.nix due to its wide usage. This cleans
up the remaining references to pkgs.pkgsconfig and
moves the entry to aliases.nix.

python3Packages.pkgconfig remained unchanged because
it's the canonical name of the upstream package
on pypi.

pkgs/applications/science/math/sage/default.nix

@@ -17,7 +17,7 @@ let
         inherit sage-src env-locations pynac singular;
         ecl = maxima-ecl.ecl;
         linbox = pkgs.linbox.override { withSage = true; };
-        pkg-config = pkgs.pkgconfig; # not to confuse with pythonPackages.pkgconfig
+        pkg-config = pkgs.pkg-config; # not to confuse with pythonPackages.pkg-config
       };
     };
   };
@@ -56,7 +56,7 @@ let
     inherit env-locations;
     inherit python3 singular palp flint pynac pythonEnv maxima-ecl;
     ecl = maxima-ecl.ecl;
-    pkg-config = pkgs.pkgconfig; # not to confuse with pythonPackages.pkgconfig
+    pkg-config = pkgs.pkg-config; # not to confuse with pythonPackages.pkg-config
   };
 
   # The documentation for sage, building it takes a lot of ram.
@@ -70,7 +70,7 @@ let
     inherit python3 pythonEnv;
     inherit sage-env;
     inherit pynac singular maxima-ecl;
-    pkg-config = pkgs.pkgconfig; # not to confuse with pythonPackages.pkgconfig
+    pkg-config = pkgs.pkg-config; # not to confuse with pythonPackages.pkg-config
     three = nodePackages.three;
   };