php: split php.packages to php.packages and php.extensions

So now we have only packages for human interaction in php.packages and
only extensions in php.extensions. With this php.packages.exts have
been merged into the same attribute set as all the other extensions to
make it flat and nice.

The nextcloud module have been updated to reflect this change as well
as the documentation.

nixos/modules/services/web-apps/nextcloud.nix

@@ -7,12 +7,12 @@ let
   fpm = config.services.phpfpm.pools.nextcloud;
 
   phpPackage = pkgs.php74.buildEnv {
-    exts = pp: with pp.exts; [
+    exts = pp: with pp; [
       bcmath calendar curl exif ftp filter gd gettext gmp intl json ldap
       mysqlnd opcache openssl pcntl pdo pdo_mysql pdo_odbc pdo_pgsql
       pdo_sqlite pgsql readline session soap sodium sqlite3 zip zlib mbstring
-      posix ctype dom simplexml xmlreader xmlwriter pp.apcu
-      pp.redis pp.memcached pp.imagick
+      posix ctype dom simplexml xmlreader xmlwriter
+      apcu redis memcached imagick
     ];
     extraConfig = phpOptionsStr;
   };