InjectionSite mapping should support multiple potential sites

Currently we only support a single injection site for each injectable attribute. However, in general, there may be multiple potential sites: for example, the attribute could be mapped to two setters with the same attribute name (e.g. same name in @Property), or it could be mapped to a constructor parameter and also to a setter (to allow for reinjection).

To support this, InjectingComponentType should be refactored to allow for multiple injection sites. It would then be the responsibility of the generator to select which sites should be used for construction, initial injection, and reinjection.