Previously, when no file annotations were present, the FIR element
didn't have a source.
By making it nullable, it will only be created when appropriate and the
source will never be null.
#KT-55835
In case of broken code e.g., duplicated classes provider would return first class,
though we definitely need some code insight in this case
at least to add navigation fixes, etc.
Similar to this, a file copy is created during completion,
where additional elements might appear and we need to search for them.
Added test cases for duplicated classes.
Kirill Rakhman