tools: add lorax-template-pkg.py helper

Add a simple helper that is meant to gather the list of packages
to be installed via a lorax template that uses the "installpkg"
directives. A prominent example is the 'runtime-install.tmpl'
script from lorax-templates-generic, used to create boot isos.

tools/lorax-template-pkgs.py

@@ -0,0 +1,64 @@
+#!/usr/bin/python3
+"""Collect to be installed packages of a lorax template script
+
+This simple tool intercepts all `installpkg` commands of a lorax
+template script like `runtime-install.tmpl` in order to collect
+all to be installed packages. The result is presented on stdout
+in form of a JSON array.
+"""
+
+import argparse
+import json
+import sys
+
+from osbuild.util.lorax import render_template
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--basearch", help="Set the `basearch` variable", default="x86_64")
+    parser.add_argument("--product", help="Set the `product` variable", default="fedora")
+    parser.add_argument("FILE", help="The template to process")
+    args = parser.parse_args()
+
+    variables = {
+      "basearch": args.basearch,
+      "product": args.product
+    }
+
+    txt = render_template(args.FILE, variables)
+
+    packages = []
+    optional = []
+    excludes = []
+
+    parser =  argparse.ArgumentParser()
+    parser.add_argument("--optional", action="append")
+    parser.add_argument("--except", dest="excludes", action="append")
+    parser.add_argument("packages", help="The template to process", nargs="*")
+
+    for line in txt:
+        cmd, args = line[0], parser.parse_args(line[1:])
+
+        if cmd != "installpkg":
+            print(f"{cmd} ignored", file=sys.stderr)
+            continue
+
+        if args.optional:
+            optional += args.optional
+        if args.excludes:
+            excludes += args.excludes
+        if args.packages:
+            packages += args.packages
+
+    data = {
+        "packages": packages,
+        "optional": optional,
+        "except": excludes
+    }
+
+    json.dump(data, sys.stdout, indent=2)
+
+
+if __name__ == "__main__":
+    main()