I'm going around in circles trying to figure this out;
I created a Java project in Eclipse called alfresco and imported the latest source bundle into it. The ant targets ran fine, but I can't get rid of the errors in Eclipse.
The first error is in the package statement in BaseConfigService.java and no matter what I set the Properties/Java Build Path/Source folders on build path to, nothing changes. The error remains "The declared package does not match the expected path projects.core.source.java.org.alfresco.config"
The alfresco source bundle is pre-packaged with eclipse projects. You can import those projects into eclipse and all should be fine. They're the projects we use ourselves.
There are several projects and each can be found in the sub-directories of /projects.
Apologies, the source folder is the way to resolve it, but it needs a wildcard specification and I had to run the default ant task to force some kind of refresh in Eclipse.
One other minor issue was that the MyEclipse plug-in was generating errors because it was looking for DTDs or schemas to validate the XML. I turned this off and all was well.