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.

.gitignore

@@ -14,8 +14,6 @@ venv/
 dist/
 build/*
 *.egg-info/
-pkg
-src/source/
 package-manager-*
 
 # Editor files