The code here is intended to walk the incoming feature, looking for
either '.' or '/' in order to break up the feature into a path and
filename. This is then compared with the load path to find a match,
indicating that the feature has been loaded via those paths.

The bug is that it should find one of those characters or leave the
search index 'e' at the beginning of the feature string for a
subsequent check, but it was ported incorrectly from the original
C code. Index of stopping at the beginning of the string, it allows
one more decrement leaving 'e' as -1, and String.charAt raises.

This is normally only triggered for features that are being
required at the same time across threads, and which have not yet
been expanded to their full load path form. With the unexpanded
feature in flight, the search will never encounter '.' or '/' and
walk off the head of the string.

Fixes jruby#8958

When a simple feature lock name is provided here, such as when a
thread has just started doing its search for a given feature, the
initial backward walk through that name used to walk past the first
character. This subsequently triggered an AIOOB exception.

The test here attempts to simulate this situation, since it's a
very tricky set of concurrent loads required to trigger it
organically.

See jruby#8958