rb_feature_p: skip `get_expanded_load_path` for absolute paths

	Ref: https://github.com/fxn/zeitwerk/pull/308

	```ruby
	require 'benchmark'

	$LOAD_PATH << 'relative-path'

	autoload :FOO, '/tmp/foo.rb'

	puts Benchmark.realtime {
	  500_000.times do
	    Object.autoload?(:FOO)
	  end
	}
	```

	The above script takes 2.5 seconds on `master`, and only
	50ms on this branch.

	When we're looking for a feature with an absolute path, we don't
	need to call the expensive `get_expanded_load_path`.