BLD: migrate package layout to src #1717

Job	Run time
Image tests	22s
type-checking (3.9)	32s
tests (ubuntu-latest, 3.9)	58s
type-checking (3.10)	29s
type-checking (3.11)	29s
tests (ubuntu-latest, 3.10)	58s
tests (ubuntu-latest, 3.11)	59s
type-checking (3.12)	35s
type-checking (3.13)	1m 54s
tests (ubuntu-latest, 3.12)	1m 10s
tests (ubuntu-latest, 3.13)	1m 26s
tests (ubuntu-20.04, 3.9, minimal, --resolution=lowest-direct)	1m 12s
tests (macos-latest, 3.13)	56s
tests (windows-latest, 3.13)	3m 0s
docs	15s
Combine coverage reports	10s
	15m 25s

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