contrib: don't build packages when generating the meson contrib files

We can just depend on the tarball. This is the smallest dependency we have. We still want to keep using dependencies so when tools are added/removed the file is properly regenerated.

Then we can bring back the generation of this file when building native tools. It will download the tools tarballs (and its dependencies), but not try to build them.

Includes !6464 (merged) that should use prebuilt without building target code on all targets now.

Draft on top of !6488 (merged)