scripted-diff: Rename PACKAGE_* variables to CLIENT_*

This change ensures consistent use of the `CLIENT_` namespace everywhere
in the repository.

-BEGIN VERIFY SCRIPT-

ren() { sed -i "s/\<$1\>/$2/g" $( git grep -l "$1" ./cmake ./src :\(exclude\)./src/secp256k1 ./test ) ; }

ren PACKAGE_NAME      CLIENT_NAME
ren PACKAGE_VERSION   CLIENT_VERSION_STRING
ren PACKAGE_URL       CLIENT_URL
ren PACKAGE_BUGREPORT CLIENT_BUGREPORT

-END VERIFY SCRIPT-

test/functional/feature_settings.py

@@ -46,7 +46,7 @@ class SettingsTest(BitcoinTestFramework):
 
         # Assert default settings file was created
         self.stop_node(0)
-        default_settings = {"_warning_": f"This file is automatically generated and updated by {self.config['environment']['PACKAGE_NAME']}. Please do not edit this file while the node is running, as any changes might be ignored or overwritten."}
+        default_settings = {"_warning_": f"This file is automatically generated and updated by {self.config['environment']['CLIENT_NAME']}. Please do not edit this file while the node is running, as any changes might be ignored or overwritten."}
         with settings.open() as fp:
             assert_equal(json.load(fp), default_settings)