flutter: move build-support/flutter to flutter directory

Rationale:

With the advent of RFC 140, Nixpkgs becomes to promote self-contained package
directories and strongly discourages scattering along the old and deprecated
filesystem hierarchy.

Further, `build-support/flutter/*` is not referenced by any other expressions in
Nixpkgs besides flutter itself.

Further, concentrating all Flutter-related toolset below a same directory
hierarchy (arguably?) eases maintenance and future migrations.

pkgs/development/compilers/flutter/default.nix

@@ -56,7 +56,7 @@ let
     (mkCustomFlutter args).overrideAttrs (prev: next: {
       passthru = next.passthru // rec {
         inherit wrapFlutter mkCustomFlutter mkFlutter;
-        buildFlutterApplication = callPackage ../../../build-support/flutter { flutter = wrapFlutter (mkCustomFlutter args); };
+        buildFlutterApplication = callPackage ./build-support/build-flutter-application.nix { flutter = wrapFlutter (mkCustomFlutter args); };
       };
     });