Use ServiceProvider for inter-module dependencies

Use ServiceProvider for inter-module dependencies

This will stop dependency chains from preventing module reloads when
it is not actually needed; however, it removes some failsafes that will
need to be reimplemented in order to avoid unmapped vtables.

This deprecates Request as an inter-module signaling mechanism, although
SQL still uses it.

git-svn-id: http://svn.inspircd.org/repository/trunk/inspircd@12140 e03df62e-2008-0410-955e-edbf42e46eb7