jh_classpath: handle multiple package tmpdirs

When reading .classpath files, jh_classpath will try to find jars by
relative location first, and fall back to using the *first* package's
tmpdir subsequently. This breaks handling for multi-binary sources where
jars appear in the second or subsequent binary package, as jars will be
searched for in the wrong tmpdir.

Fix this by using the tmpdir corresponding to $package.