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).

[package]
name = "libpam-sys-helpers"
description = "Cross-platform helpers for libpam-sys."
version.workspace = true
authors.workspace = true
repository.workspace = true
edition.workspace = true
rust-version.workspace = true
license.workspace = true
readme = "README.md"

[build-dependencies]
libpam-sys-impls = { version = "0.1.0", path = "../libpam-sys-impls" }