Provides support for extending the base installation and update services.

## Package Specification

This package specifies the API required for writing pluggable support of alternate feature packaging schemes and update site access mechanisms. For the most part, the classes and interfaces in this package implement a code pattern `class Foo extends FooModel implements IFoo`. The interface (`IFoo` in the pattern) defines the actual API. The convenience classes (`Foo` in the pattern) provide a reference implementation of the API based on a model (`IFooModel` in the pattern) contained in package org.eclipse.update.core.model. In general, developers should extend the convenience classes rather than completely reimplementing the specified interfaces.