This allows us to get away without whole-archive in CMake by declaring
an OBJECT library instead.

Otherwise, Ninja will complain, as it needs **some** rule to build the
target, even if that rule does nothing.

This caused a build failure when not building a binary.
Linking the object file in shouldn't produce issues.

And don't redirect the first build output to /dev/null

This should allow us to see what exactly is failing in CI.

builders

Otherwise the linker order doesn't work out and the linker cannot
resolve the qt_static_plugin_*** functions

It's much more concise than using File::create followed by write!

This can be used to e.g. register custom types, import plugins, declare
modules, etc.

This reverts commit 40f5ad1.

We are now building and linking object files directly, which works
nicely with CMake and seems to work for Rust as well.

This may be a clippy bug, as `cargo expand` tells me the generated
functions do indeed have a `# Safety` section in their documentation.

The `# Safety` sections also show up in the output of `cargo doc`.
So it's unclear why clippy is complaining.

This way we don't need a separate object file and CMake target for this.