Refine packaging layout and Arch build paths

* Move Arch-specific ignore rules into `packaging/arch/.gitignore` and simplify top-level `.gitignore`/`.dockerignore`.
* Update Arch `PKGBUILD` to sync from the project root and drop `packaging/` from the installed tree.
* Fix OS-specific `package.sh` helpers to resolve the new `packaging/*` locations correctly for Arch, Debian/Ubuntu, Fedora, and CentOS.

packaging/arch/.gitignore

@@ -0,0 +1,6 @@
+# Arch pkg artifacts
+*.pkg.tar.*
+*.log
+package-manager-*
+src/
+pkg/