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/src/lib.rs	Thu Jul 31 15:42:12 2025 -0400
+++ b/src/lib.rs	Sat Aug 02 18:47:46 2025 -0400
@@ -257,4 +257,6 @@
     handle::{ModuleClient, PamShared, Transaction},
     module::PamModule,
 };
-use libpam_sys_impls::pam_impl_name;
+#[doc(inline)]
+pub use libpam_sys::pam_impl;
+use libpam_sys::pam_impl_name;