Fix the usage: detect script languages correctly

We have this incredibly cute code that verifies that certain files are
available before reporting to support certain script languages.

It got broken when we switched to versioned files in jars/ and nobody
noticed.

Until now. I noticed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'fix-usage'

This topic fixes a long-standing bug with our script-language-specific
usage output.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>