Issue1205206

When the jar file in packages.idx exists, but the cache 
file path that it maps to is bad, jython complains that it 
cannot write a new cachefile.

This fix causes jython to regenerate the path for the 
cachefile (using cachedir and the jar filename) rather 
than reusing the bad path in the .idx file.

The code assumed that if a jar existed in the .idx, its 
associated cachefile path should be reused to write a 
new cachefile.  This is a bad assumption if you run 
jython on the same set of jars, but your cachedir is in 
an automounted directory.  The automounted directory 
can be different depending upon the machine you are 
running from