Rename ClassPath.findSourceFile and delete Platform.doLoad

This is a change cherry-picked from a branch with new classpath implementation as
it's something independent which doesn't affect anything and so that it should be safe to merge it.

If you look at the implementation of ClassPath.findSourceFile method and its usage
it's clear that it should have been named `findClassFile` from the
beginning because that's what it does: find a class file and not a source file.

We don't need Platform.doLoad - .NET backend has been removed so method is a no-op.