I\'ve downloaded the source for all the Maven JARs in my buildpath using Maven->Download Sources and mvn eclipse:eclipse -DdownloadSources=true, which worked! W
I found this com.ifedorenko.m2e.sourcelookup plugin. The link was provided in "Java Debugging: Source not found".
Go to Help > Install New Software .. and use
http://ifedorenko.github.com/m2e-extras/
to search for the links. After installing this plugin I was finally able to debug the source code of dependencies.