The logic here was ported from CRuby, which only considers .rb to
be a source extension. This causes our logic to think a filename
like "foo.class" is not a source file and try to load it as either
"foo.class.rb" or "foo.class.class". This is the cause of jruby#8758 and
prevents explicitly loading precompiled Ruby sources using the
filename with .class extension.

The fix here modifies the source extension check to also consider
other configured source extensions, so that when .class searching
is enabled, we will properly treat `require "foo.class"` as a
source file.

Fixes jruby#8758

This enables precompiled .class files to be loaded with require,
since that behavior was made optional in JRuby 9.3, and adds a
test that confirms it works for a full filename, reported as
broken in jruby#8758.