This is pretty much a copy of the final ExtensionFinder.Sezpoz class from 1.651 fitted
for a custom ClassLoader rather than checking Jenkins
The only differences are:
* getIndices -> ClassLoader parameter; doesn't check Jenkins
* find -> ClassLoader parameter
* scout -> ClassLoader parameter
IMPORTANT: don't use find(Class<T> type, Hudson hud) as the getIndices method will error.