Fix URL filtering when loading from a directory

The ExplodedArchive would erroneously always attempt to filter
its contents (and thereby shield them from a classloader that wrapped
it) even if they haven't been explicitly provided.

See gh-1352
Normalize search path in PropertiesLauncher

* Windows: allow absolute file paths without file:/// prefix
* All: only add nested archives (not directories), so loader.path=lib/*
behaves the same as -classpath=lib/* (except for adding zip files)

Fixes gh-1352