Move code_context_extractor.py and unused_definition_remover.py from
codeflash/context/ to codeflash/languages/python/context/ and update
all import sites.
Assignments that don't reference module-level definitions are now placed
right after imports. Only assignments that reference classes/functions
are placed after those definitions to prevent NameError.
Update test_no_targets_found to expect outer class to be kept when
targeting nested class methods, and add test for nonexistent targets.
Update test_multi_file_replcement01 to expect global assignments at
module end rather than after imports.
