Move translations to new paths

We ship packaged locales for all the locales we support, and we do not support langpacks anymore.

Therefore, we can start using Firefox l10n directories, and this removes the need for additional changes to browser/installer/package-manifest.in, but also a few tweaks on projects/firefox/build.

We will need it to finish with Crypto Safety prompt for tor-browser#41351 (closed), but we could do the same for other components.