php.packages: Use derivations for internalDeps in mkExtension

Make mkExtension put headers in the dev output and use them, instead of
a different part of the current source tree, when referring to another
extension by using internalDeps.

This means external extensions can be built against the internal ones.

pkgs/development/interpreters/php/default.nix

@@ -171,7 +171,8 @@ let
                 in
                   lib.nameValuePair extName {
                     text = "${type}=${ext}/lib/php/extensions/${extName}.so";
-                    deps = lib.optionals (ext ? internalDeps) ext.internalDeps;
+                    deps = lib.optionals (ext ? internalDeps)
+                      (map getExtName ext.internalDeps);
                   })
                 extList);