Until now, if e.g. the user passed "en_US.UTF-8" instead of "en_US.UTF-8/UTF-8",
the locales would be generated without failing but wouldn't work well.
Now we guard against such mistakes. Real life examples:
https://github.com/fish-shell/fish-shell/issues/1927
- there were many easy merge conflicts
- cc-wrapper needed nontrivial changes
Many other problems might've been created by interaction of the branches,
but stdenv and a few other packages build fine now.
