Bug 6672 jar no longer exported after attach source
1206 1. Create a project test. 2. Add a jar (say resources.jar) to the classpath and export the entry 3. Attach source (say resourcessrc.zip) to the jar. Notice, the jar is no longer exported - this information is discarded during the attach source process.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/SourceAttachmentBlock.java
/**
	 * Takes a path and replaces the beginning with a variable name
	 * (if the beginning matches with the variables value)
	 */
	private IPath modifyPath(IPath path, String varName) {
		if (varName == null || path == null) {
			return null;
		}
		if (path.isEmpty()) {
			return new Path(varName);
		}
		
		IPath varPath= JavaCore.getClasspathVariable(varName);
		if (varPath != null) {
			if (varPath.isPrefixOf(path)) {
				path= path.removeFirstSegments(varPath.segmentCount());
			} else {
				path= new Path(path.lastSegment());
			}
		} else {
			path= new Path(path.lastSegment());
		}
		return new Path(varName).append(path);
	}
	
	
	/**
	 * Creates a runnable that sets the source attachment by modifying the project's classpath.
	 */