Make sure that when loading from jar, $LOADED_FEATURES includes full path.

Also renamed path -> entryPath in hope to avoid similar bugs in future.
Have an explicit test about allowing both test.jar!/foo.rb and test.jar!foo.rb
Avoid to have to do premature string concatenation.

Technically previously we allowed to lookup some.jar!foo.rb if somejar.!/foo.rb existed, but since creating a real jar with /foo.rb is hard and uncommon, dropping the feature should not have any consequences.
Extract when paths should bypass load paths into a special helper to clarify logic.
Merge pull request #1848 from ratnikov/test-fix-jars

Get jruby-jars to build correctly