I think this is safe, judging by the integration tests, but I'm not
putting it in 1.2.x until we've had some feedback on it. The
integration tests actually had a bug that was masking this problem
because they were merging Properties from the whole classpath instead
of picking the first available resource (which is generally what
we do in Spring Boot applications for application.properties for
instance).
Fixes gh-3048
If user runs an executable archive then it and its lib directory will be
on the classpath. Entries from loader.path take precedence in a way that
should make sense to users (earlier wins like in CLASSPATH env var).
Also added new integration tests to verify the behaviour (big improvement
on the old ones, which probably aought to be beefed up to the same
standard).
Fixes gh-2314
