Rework mod build system to be multi-version capable

Add a variants array which can specify overriden properties of the
info.json for building multiple versions of the mod at the same time and
fix the additonal_files to allow specifying under what path the file is
put.

Together these two changes allow setting up the build to do all
supported versions at the same time and add specialized code for each
version of Factorio supported.

packages/slave/lua/clusterio_lib/info.json

@@ -1,12 +1,20 @@
 {
     "name": "clusterio_lib",
-    "version": "0.1.1",
+    "variants": [
+        {
+            "version": "0.1.0",
+            "factorio_version": "0.17"
+        },
+        {
+            "version": "0.1.1",
+            "factorio_version": "0.18"
+        }
+    ],
     "title": "Clusterio Library (Alpha)",
     "author": "Clusterio Team",
     "homepage": "https://github.com/clusterio/factorioClusterio",
     "description": "Library for interfacing with Clusterio.  Warning: work in progress alpha.",
-    "factorio_version": "0.17",
-    "additional_files": [
-        ["..", "..", "modules", "clusterio", "serialize.lua"]
-    ]
+    "additional_files": {
+        "serialize.lua": ["..", "..", "modules", "clusterio", "serialize.lua"]
+    }
 }