They are added to installable_targets and can optionally be
included in a MULTIPROCESS=1 custom Guix build. This commit
ensures we run the binary security, symbol and dynamic library
checks on them (when present).
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_*`.
