build: Rename PACKAGE_* variables to CLIENT_*

The use of `PACKAGE_NAME` for the project's variable name is
problematic, as this name is commonly used in CMake's interface
variables. If third-party CMake code handles with scopes improperly,
our `PACKAGE_NAME` variable could end up with an unexpected value.

This change avoids such conflicts by renaming all `PACKAGE_*` variables
to `CLIENT_*`.

test/config.ini.in

@@ -6,8 +6,8 @@
 # test/*/test_runner.py and test/util/rpcauth-test.py
 
 [environment]
-PACKAGE_NAME=@PACKAGE_NAME@
-PACKAGE_BUGREPORT=@PACKAGE_BUGREPORT@
+PACKAGE_NAME=@CLIENT_NAME@
+PACKAGE_BUGREPORT=@CLIENT_BUGREPORT@
 SRCDIR=@abs_top_srcdir@
 BUILDDIR=@abs_top_builddir@
 EXEEXT=@EXEEXT@