Rename ClassPath.findSourceFile to ClassPath.findClassFile

If you look at the implementation of that method and its usage
its 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.
Get rid of Platform.doLoad method.

Since .NET backend got removed this method is a no-op.
Merge pull request #3999 from mpociecha/classpath-findSourceFile-method-renaming

Rename ClassPath.findSourceFile and delete Platform.doLoad