Add support for making “temporary” queries

[progval]

I don't know how to name it better.

Basically, I would like to be able to write a module that performs operations on data returned by another module, without this other data being displayed to the user.
Example: if I want to write a module that answers (Donald Knuth,age,?). This module would ask other modules (the core?) for (Donald Knuth,birth date,?), get the answer, and make a substraction.

The only way for now is to hardcode the URL to the core in the module's config. It would be nice to have another way. (I don't like cyclic graphs.)