stubgen: fix FunctionContext.fullname for nested classes #13769

Job	Run time
Test mypyc suite with 32-bit Python	7m 12s
Test suite with py38-ubuntu, mypyc-compiled	17m 42s
Test suite with py38-windows-64	19m 37s
Test suite with py39-ubuntu	16m 44s
Test suite with py310-ubuntu	16m 8s
Test suite with py311-ubuntu, mypyc-compiled	15m 38s
Test suite with py312-ubuntu, mypyc-compiled	18m 48s
Test suite with py313-ubuntu, mypyc-compiled	18m 28s
mypyc runtime tests with py39-macos	5m 9s
Type check our own code (py38-ubuntu)	1m 58s
Type check our own code (py38-windows-64)	2m 34s
Formatting and code style with Black + ruff	35s
	2h 20m 33s

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