Eclipse has absolutely zero to do with running a JSF webapp as far as cross-referencing projects are concerned. No matter what you define in Eclipse, it won't get referenced come runtime. The only things that your webapp container and the webapp itself can locate are items in the deployed application or in the webapp container's CLASSPATH chains.
On the other hand, if what you really mean is that you have created some sort of component that you want to deploy as part of a webapp, but the component is constructed outside of the project where you're building the webapp itself, you need to tailor the webapp build process so that the constructed component will be copied into the resulting deployable
unit. In other words, if I made a Google Maps JSF tag component (which actually, I have) and made it a drop-in component in a JAR, I would copy that JAR into my webapp build. If the actual Google Maps component JAR is built in a different Eclipse project, about the only things Eclipse will do for me is allow me to get at the internal symbols for debugging purposes and ensure that any automated dependent build processes get done in the proper order (most of my deployables are built with
Maven or Ant, so that's of limited use to me).