qt5: Expose internal variables to facilitate out-of-tree building of modules

As it is currently it's almost impossible to accomplish something like:
```nix
...

(qt5.callPackage (
  { qtModule, qtbase, qtdeclarative }:
  qtModule {
    pname = "qt3d";
    qtInputs = [ qtbase qtdeclarative ];
    outputs = [ "out" "dev" "bin" ];
  }
) { })

...
```
because all the required variables such as qtModule are internal-only.

By exposing these it's easier for external projects not to be bound
exactly by the module definitions explicitly listed in nixpkgs.

pkgs/development/libraries/qt-5/5.12/default.nix

@@ -127,6 +127,8 @@ let
       callPackage = self.newScope { inherit qtCompatVersion qtModule srcs; };
     in {
 
+      inherit callPackage qtCompatVersion qtModule srcs;
+
       mkDerivationWith =
         import ../mkDerivation.nix
         { inherit lib; inherit debug; inherit (self) wrapQtAppsHook; };

pkgs/development/libraries/qt-5/5.14/default.nix

@@ -139,6 +139,8 @@ let
       callPackage = self.newScope { inherit qtCompatVersion qtModule srcs; };
     in {
 
+      inherit callPackage qtCompatVersion qtModule srcs;
+
       mkDerivationWith =
         import ../mkDerivation.nix
         { inherit lib; inherit debug; inherit (self) wrapQtAppsHook; };

pkgs/development/libraries/qt-5/5.15/default.nix

@@ -165,6 +165,8 @@ let
       callPackage = self.newScope { inherit qtCompatVersion qtModule srcs; };
     in {
 
+      inherit callPackage qtCompatVersion qtModule srcs;
+
       mkDerivationWith =
         import ../mkDerivation.nix
         { inherit lib; inherit debug; inherit (self) wrapQtAppsHook; };