ScriptFinder: fix section header
WIP: ScriptFinder: discover each script only once

Even if overlapping base script directories are added, such that a
script might be examined more than once, let's only add it to the first
time it is matched.

This will be useful to support Fiji 1.x's special handling of scripts in
the Plugins/Scripts subtree.
WIP: ScriptFinderTest: add test for overlapping dirs

Still to do:
- imagej-legacy: scriptService.addScriptDirectory on legacy script paths