Bug 33289: Optimize the get_target function

We also change the behavior of the function: we stop looking after the first definition of a target. This means for example that if a target is defined as an array in rbm.conf, and also defined as a hash in the project's config, then the definition from rbm.conf will be ignored.

In tor-browser-build we have a few targets defined as an array (for example torbrowser-linux-x86_64 is an array containing linux-x86_64 and linux), but we never redefine those targets in project configs, so this change of behavior should not affect tor-browser-build.

Closes #33289 (closed).