BLD: migrate package layout to src #1716

Job	Run time
tests (ubuntu-latest, 3.9, bin)	55s
type-checking (3.9)	33s
tests (ubuntu-latest, 3.10, bin)	59s
Image tests	19s
type-checking (3.10)	33s
tests (ubuntu-latest, 3.11, bin)	55s
type-checking (3.11)	32s
tests (ubuntu-latest, 3.12, bin)	1m 13s
type-checking (3.12)	36s
tests (ubuntu-latest, 3.13, bin)	1m 33s
type-checking (3.13)	2m 0s
tests (ubuntu-20.04, 3.9, bin, minimal, --resolution=lowest-direct)	1m 14s
tests (macos-latest, 3.13, bin)	1m 3s
tests (windows-latest, 3.13, Scripts)	2m 48s
docs	14s
Combine coverage reports	10s
	15m 37s

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