Restructure the way libpam-sys-impls works to fix cross-compilation. The previous structure of libpam-sys-impls meant that things got confusing (including for me) between what constants were build-time and what constants were run-time. This broke cross-compilation. This simplifies the way that works so that `libpam-sys-impls` has *no* build script itself and is intended mostly as a library to be included in other libraries' build scripts (while also exporting the PamImpl enum).

--- a/libpam-sys/src/constants.rs	Thu Jul 31 15:42:12 2025 -0400
+++ b/libpam-sys/src/constants.rs	Sat Aug 02 18:47:46 2025 -0400
@@ -172,6 +172,7 @@
         }
     }
 
+    #[doc(inline)]
     pub use pam_modutil_redirect_fd::*;
 }