build: distribute gresource c/h with source tarball

This introduces the concept of a "dist resource" (specifically a
`GhosttyDist.Resource` type). This is a resource that may be present in
dist tarballs but not in the source tree. If the resource is present and
we're not in a Git checkout, then we use it directly instead of
generating it.

This is used for the first time in this commit for the gresource c/h
files, which depend on a variety of external tools (blueprint-compiler,
glib-compile-resources, etc.) that we do not want to require downstream
users/packagers to have and we also do not want to worry about them
having the right versions.

This also adds a check for `distcheck` to ensure our distribution
contains all the expected files.

nix/devShell.nix

@@ -108,6 +108,12 @@ in
 
         # Localization
         gettext
+
+        # We need these GTK-related deps on all platform so we can build
+        # dist tarballs.
+        blueprint-compiler
+        libadwaita
+        gtk4
       ]
       ++ lib.optionals stdenv.hostPlatform.isLinux [
         # My nix shell environment installs the non-interactive version
@@ -146,9 +152,6 @@ in
         libXrandr
 
         # Only needed for GTK builds
-        blueprint-compiler
-        libadwaita
-        gtk4
         gtk4-layer-shell
         glib
         gobject-introspection