Ant detects non-existent package while IDE shows error

Ant detects non-existent package while IDE shows error

My ant build is working fine (compile, jar goals execute without errors).
However, the IDE shows errors in the code. I have tried Eclipse and
IntelliJ-IDEA; both show the same behaviour.
For example, in the following line
import com.google.javascript.rhino.head.ErrorReporter;
my IDE points out that the package com.google.javascript.rhino.head does
not exist. And it is true as I verified that no such package exists at
that location.
I would like to configure my IDE to build correctly so that I can use
debug features. What am I missing here?