[Extensions] Add SetExtraEntryPoints to Extension C API

This will allow for Extension to inform if they have other entry
points (apart from the implicit extension_name), that should cause
the extension to be loaded.

For example, the "calendar" extension may export some constants in the
CalendarAppointment object, and we want the calendar extension to be
loaded when any of these "main" objects are accessed.

An additional internal API header is introduced to keep this new API
contained, until this idea is proven.
[Extensions] Add tests for loading extension with multiple entry points

This test verifies that the extension is loaded even if the entry point
is in a different object than the one implicitly created by the extension
name.
[Extensions] Verify that the entry points are valid names

Also make sure that there aren't any extension already registered
with clashing names (or entry points).

We re-use the function that used to validate extension names to validate
entry points as well, so we rename it to something more appropriate.
[Extensions] Add tests for conflicts between extensions

This adds for when an extension adds an entry point for an object
that belongs to another extension, either the extension name or
via entry points.