stubgen: multiple fixes to the generated imports #11188

Job	Run time
Test mypyc suite with 32-bit Python	11m 46s
Test suite with py38-ubuntu, mypyc-compiled	24m 3s
Test suite with py38-windows-64	32m 53s
Test suite with py39-ubuntu	23m 2s
Test suite with py310-ubuntu	29m 38s
Test suite with py311-ubuntu, mypyc-compiled	21m 16s
Test suite with py312-ubuntu, mypyc-compiled	23m 57s
mypyc runtime tests with py38-macos	10m 2s
mypyc runtime tests with py38-debug-build-ubuntu	6m 48s
Type check our own code (py38-ubuntu)	2m 23s
Type check our own code (py38-windows-64)	3m 6s
Formatting and code style with Black + ruff	40s
	3h 9m 34s

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