stubgen: fix FunctionContext.fullname for nested classes #13762

Job	Run time
Test mypyc suite with 32-bit Python	7m 17s
Test suite with py38-ubuntu, mypyc-compiled	1m 38s
Test suite with py38-windows-64	20m 4s
Test suite with py39-ubuntu	16m 55s
Test suite with py310-ubuntu	15m 52s
Test suite with py311-ubuntu, mypyc-compiled	1m 13s
Test suite with py312-ubuntu, mypyc-compiled	1m 18s
Test suite with py313-ubuntu, mypyc-compiled	1m 28s
mypyc runtime tests with py39-macos	6m 31s
Type check our own code (py38-ubuntu)	1m 53s
Type check our own code (py38-windows-64)	2m 27s
Formatting and code style with Black + ruff	33s
	1h 17m 9s

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