Fix multi-package build: Rename packages to match source name

- Rename all packages from 'deb-mock-*' to 'mock-*' to match source name 'mock'
- Update debian/control package definitions and dependencies
- Rename .install files to match new package names
- Update CI workflow to look for 'mock_*.deb' instead of 'deb-mock_*.deb'

This fixes the core issue where only 1 package was being built instead of 6.
The Debian build system now correctly recognizes all 6 packages:
- mock (main package)
- mock-cache (cache utilities)
- mock-configs (configuration files)
- mock-dev (development tools)
- mock-filesystem (filesystem layout)
- mock-plugins (plugin system)

All 6 packages now build successfully locally and should work in CI.

debian/mock.postinst.debhelper

@@ -0,0 +1,10 @@
+
+# Automatically added by dh_python3
+if command -v py3compile >/dev/null 2>&1; then
+	py3compile -p mock 
+fi
+if command -v pypy3compile >/dev/null 2>&1; then
+	pypy3compile -p mock  || true
+fi
+
+# End automatically added section