Limit the choice of the targets argument to target we know about.

By limiting the choices in the argument parser, we catch argument error
at parsing and we can provide useful output (the names of dependencies
we can handle) to user in the help or in the error message.

kiwix-build.py

@@ -780,7 +780,8 @@ class Builder:
 
 def parse_args():
     parser = argparse.ArgumentParser()
-    parser.add_argument('targets', default='kiwix-tools', nargs='?')
+    parser.add_argument('targets', default='kiwix-tools', nargs='?',
+                        choices=Dependency.all_deps.keys())
     parser.add_argument('--working-dir', default=".")
     parser.add_argument('--libprefix', default=None)
     parser.add_argument('--target-platform', default="native_dyn", choices=BuildEnv.target_platforms)