Without this patch, setting the same locale twice, e.g. like this in
NixOS:
```nix
{
i18n.supportedLocales = [
(config.i18n.defaultLocale + "/UTF-8")
(config.i18n.defaultLocale + "/UTF-8")
];
}
```
Would make the glibcLocales build fail with
```
Error: unsupported locales detected:
en_US.UTF-8/UTF-8 \
You should choose from the list above the error.
```
Previously we only build C.UTF-8 as part of the locale-archive
that comes with the glibc core package.
However for consistent use of LANG=C.UTF-8 we also want support
in our glibcLocales as well.
fixes https://github.com/NixOS/nixpkgs/issues/57974
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.
