[packages] Document and add new key to schema

FIXES #1676

src/modules/packages/packages.conf

@@ -1,13 +1,30 @@
 # SPDX-FileCopyrightText: no
 # SPDX-License-Identifier: CC0-1.0
 #
+# The configuration for the package manager starts with the
+# *backend* key, which picks one of the backends to use.
+# In `main.py` there is a base class `PackageManager`.
+# Implementations must subclass that and set a (class-level)
+# property *backend* to the name of the backend (e.g. "dummy").
+# That property is used to match against the *backend* key here.
+#
+# You will have to add such a class for your package manager.
+# It is fairly simple Python code. The API is described in the
+# abstract methods in class `PackageManager`. Mostly, the only
+# trick is to figure out the correct commands to use, and in particular,
+# whether additional switches are required or not. Some package managers
+# have more installer-friendly defaults than others, e.g., DNF requires
+# passing --disablerepo=* -C to allow removing packages without Internet
+# connectivity, and it also returns an error exit code if the package did
+# not exist to begin with.
 ---
 #
 # Which package manager to use, options are:
 #  - apk         - Alpine Linux package manager
 #  - apt         - APT frontend for DEB and RPM
 #  - dnf         - DNF, the new RPM frontend
-#  - entropy     - Sabayon package manager
+#  - entropy     - Sabayon package manager (is being deprecated)
+#  - luet        - Sabayon package manager (next-gen)
 #  - packagekit  - PackageKit CLI tool
 #  - pacman      - Pacman
 #  - pamac       - Manjaro package manager

src/modules/packages/packages.schema.yaml

@@ -13,6 +13,7 @@ properties:
             - apt
             - dnf
             - entropy
+            - luet
             - packagekit
             - pacman
             - pamac